diff --git a/.sqlx/query-6eaeee584dc98b28b9605faabab0eecf7839b7755c956731617b475921481701.json b/.sqlx/query-3d48b78768bf8dc2e337d0935889a5654089f38bb7ee2a15b7e19f4ed5540262.json
similarity index 71%
rename from .sqlx/query-6eaeee584dc98b28b9605faabab0eecf7839b7755c956731617b475921481701.json
rename to .sqlx/query-3d48b78768bf8dc2e337d0935889a5654089f38bb7ee2a15b7e19f4ed5540262.json
index 6d94a9b..b32d119 100644
--- a/.sqlx/query-6eaeee584dc98b28b9605faabab0eecf7839b7755c956731617b475921481701.json
+++ b/.sqlx/query-3d48b78768bf8dc2e337d0935889a5654089f38bb7ee2a15b7e19f4ed5540262.json
@@ -1,6 +1,6 @@
 {
   "db_name": "SQLite",
-  "query": "INSERT INTO subtitle_files\n               (media_file_id, language, origin, provider, candidate_id, engine,\n                forced, sdh, synced, sync_rejected, path)\n           VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)\n           ON CONFLICT DO NOTHING\n           RETURNING id AS \"id!: i64\"",
+  "query": "INSERT INTO subtitle_files\n               (media_file_id, language, origin, provider, candidate_id, engine,\n                forced, sdh, synced, sync_rejected, path)\n           VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)\n           ON CONFLICT (path) DO NOTHING\n           ON CONFLICT (media_file_id, language, forced, sdh)\n               WHERE origin = 'embedded' DO NOTHING\n           RETURNING id AS \"id!: i64\"",
   "describe": {
     "columns": [
       {
@@ -22,5 +22,5 @@
       null
     ]
   },
-  "hash": "6eaeee584dc98b28b9605faabab0eecf7839b7755c956731617b475921481701"
+  "hash": "3d48b78768bf8dc2e337d0935889a5654089f38bb7ee2a15b7e19f4ed5540262"
 }
diff --git a/.sqlx/query-e17b71f67d8b79c54d2d6f32d5e4197d77faac1763b360e8e609b0b9ba5820f1.json b/.sqlx/query-e17b71f67d8b79c54d2d6f32d5e4197d77faac1763b360e8e609b0b9ba5820f1.json
new file mode 100644
index 0000000..b3a5a5a
--- /dev/null
+++ b/.sqlx/query-e17b71f67d8b79c54d2d6f32d5e4197d77faac1763b360e8e609b0b9ba5820f1.json
@@ -0,0 +1,26 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT path AS \"path!: String\" FROM subtitle_files\n           WHERE  media_file_id = ? AND language = ? AND path IS NOT NULL",
+  "describe": {
+    "columns": [
+      {
+        "name": "path!: String",
+        "ordinal": 0,
+        "type_info": "Text",
+        "origin": {
+          "Table": {
+            "table": "subtitle_files",
+            "name": "path"
+          }
+        }
+      }
+    ],
+    "parameters": {
+      "Right": 2
+    },
+    "nullable": [
+      true
+    ]
+  },
+  "hash": "e17b71f67d8b79c54d2d6f32d5e4197d77faac1763b360e8e609b0b9ba5820f1"
+}
diff --git a/crates/arr-db/migrations/0029_one_sidecar_per_language.sql b/crates/arr-db/migrations/0029_one_sidecar_per_language.sql
new file mode 100644
index 0000000..84d44c8
--- /dev/null
+++ b/crates/arr-db/migrations/0029_one_sidecar_per_language.sql
@@ -0,0 +1,41 @@
+-- #222. DESIGN.md §15, as amended: a language is satisfied by exactly one
+-- sidecar. No filename segment distinguishes forced from plain from SDH, so
+-- two sidecar rows for one (media file, language) describe two files
+-- competing for one name. The schema says so now rather than leaving it to
+-- the API's path check.
+--
+-- Embedded rows are deliberately untouched. They describe tracks inside the
+-- video, not files on disk, and several can legitimately coexist for one
+-- language — a plain track and a forced one, say. Their key stays
+-- `(media_file_id, language, forced, sdh)`.
+
+-- A database that predates the constraint may already hold a duplicate from
+-- a manual grab that beat the path check: a fetched `.pt-PT.srt` beside a
+-- translated `.pt-PT.mt.srt`, for instance. Resolve rather than fail. Of the
+-- rows for one (media file, language): keep a real subtitle over a machine
+-- translation, and of two of the same kind the newest.
+--
+-- The files stay on disk. Deleting a viewer's subtitle during a migration is
+-- worse than leaving an orphan, and the manual delete (#218, #223) cleans one
+-- up on request.
+DELETE FROM subtitle_files
+WHERE  path IS NOT NULL
+  AND  id NOT IN (
+           SELECT id
+           FROM   (
+               SELECT id,
+                      ROW_NUMBER() OVER (
+                          PARTITION BY media_file_id, language
+                          ORDER BY (origin = 'translated') ASC,
+                                   created_at DESC,
+                                   id DESC
+                      ) AS place
+               FROM   subtitle_files
+               WHERE  path IS NOT NULL
+           )
+           WHERE place = 1
+       );
+
+CREATE UNIQUE INDEX subtitle_files_one_sidecar
+    ON subtitle_files (media_file_id, language)
+    WHERE path IS NOT NULL;
diff --git a/crates/arr-db/src/subtitles.rs b/crates/arr-db/src/subtitles.rs
index e93176b..4625481 100644
--- a/crates/arr-db/src/subtitles.rs
+++ b/crates/arr-db/src/subtitles.rs
@@ -210,15 +210,22 @@ impl NewSubtitleFile {
 
 /// Record a subtitle, returning its row id.
 ///
-/// Idempotent on both keys the schema carries: the sidecar path, and the
-/// language an embedded track satisfies. A second probe of the same video, or
-/// a re-import that finds the same sidecar, converges instead of duplicating.
-/// The first row wins — §15 has no upgrade loop, and replacing a subtitle is
-/// a manual action that deletes first.
+/// Idempotent on the two keys that mean "arr already knows this": the sidecar
+/// path, and the language an embedded track satisfies. A second probe of the
+/// same video, or a re-import that finds the same sidecar, converges instead
+/// of duplicating. The first row wins — §15 has no upgrade loop, and
+/// replacing a subtitle is a manual action that deletes first.
+///
+/// The third key is not swallowed. §15 gives a language exactly one sidecar,
+/// so a *different* file claiming a language that already has one is a real
+/// conflict — the caller wrote a second subtitle where only one may live —
+/// and the unique violation is returned rather than resolved to some other
+/// row's id. Callers that can refuse before writing anything do (#222).
 ///
 /// # Errors
 ///
-/// If the insert fails.
+/// If the insert fails, including when the one-sidecar-per-language
+/// invariant rejects it.
 pub async fn record_file(
     pool: &SqlitePool,
     subtitle: &NewSubtitleFile,
@@ -230,7 +237,9 @@ pub async fn record_file(
                (media_file_id, language, origin, provider, candidate_id, engine,
                 forced, sdh, synced, sync_rejected, path)
            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
-           ON CONFLICT DO NOTHING
+           ON CONFLICT (path) DO NOTHING
+           ON CONFLICT (media_file_id, language, forced, sdh)
+               WHERE origin = 'embedded' DO NOTHING
            RETURNING id AS "id!: i64""#,
         subtitle.media_file_id,
         subtitle.language,
@@ -251,8 +260,9 @@ pub async fn record_file(
         return Ok(id);
     }
 
-    // The insert hit one of the two unique keys. Which one is decided by the
-    // origin, because only an embedded row has no path.
+    // The insert hit one of the two keys named above — the one-sidecar
+    // index is not among them and would have raised. Which of the two is
+    // decided by the origin, because only an embedded row has no path.
     if let Some(path) = subtitle.path.as_deref() {
         sqlx::query_scalar!(
             r#"SELECT id AS "id!: i64" FROM subtitle_files WHERE path = ?"#,
@@ -323,6 +333,32 @@ pub async fn files_for(
         .collect())
 }
 
+/// The one sidecar this media file already has in `language`, if any (§15).
+///
+/// The same invariant the schema enforces, read instead of tripped over: a
+/// caller about to fetch or translate a second subtitle for a language asks
+/// this first, so it refuses before anything reaches the disk rather than
+/// writing a file the insert then rejects. Embedded rows are not sidecars and
+/// never answer here.
+///
+/// # Errors
+///
+/// If the query fails.
+pub async fn sidecar_for(
+    pool: &SqlitePool,
+    media_file_id: i64,
+    language: &str,
+) -> Result<Option<String>, sqlx::Error> {
+    sqlx::query_scalar!(
+        r#"SELECT path AS "path!: String" FROM subtitle_files
+           WHERE  media_file_id = ? AND language = ? AND path IS NOT NULL"#,
+        media_file_id,
+        language
+    )
+    .fetch_optional(pool)
+    .await
+}
+
 /// Record what `alass` did to a subtitle already on disk.
 ///
 /// A rejected sync flags the row (§15); it never deletes it, because the
@@ -582,7 +618,8 @@ pub async fn pending(pool: &SqlitePool, limit: i64) -> Result<Vec<PendingSubtitl
 mod tests {
     use super::{
         attempts_for, delete_file, files_for, mark_satisfied, mark_synced, pending, record_attempt,
-        record_file, want, NewSubtitleFile, SubtitleOrigin, SubtitleState, SubtitleSync,
+        record_file, sidecar_for, want, NewSubtitleFile, SubtitleOrigin, SubtitleState,
+        SubtitleSync,
     };
     use crate::Db;
     use sqlx::SqlitePool;
@@ -650,7 +687,10 @@ mod tests {
         let again = record_file(db.pool(), &fetched).await.unwrap();
         assert_eq!(first, again);
 
-        let translated = NewSubtitleFile::translated(file, "pt-PT", "deepl", "/m/b.pt-PT.mt.srt")
+        // A different language: §15 gives pt-PT exactly one sidecar, so the
+        // machine translation that sits beside this one is `en`, not a
+        // second pt-PT (#222).
+        let translated = NewSubtitleFile::translated(file, "en", "deepl", "/m/b.en.mt.srt")
             .sync(SubtitleSync::Synced);
         record_file(db.pool(), &translated).await.unwrap();
 
@@ -671,6 +711,222 @@ mod tests {
         assert_eq!(machine.sync, SubtitleSync::Synced);
     }
 
+    /// §15, as amended: a language is satisfied by exactly one sidecar, so a
+    /// second file claiming one is refused by the schema rather than resolved
+    /// to the first row's id (#222).
+    #[tokio::test]
+    async fn a_language_gets_exactly_one_sidecar() {
+        let (db, _dir) = database().await;
+        let file = media_file(db.pool(), "/m/one.mkv", "2026-01-01T00:00:00.000Z").await;
+
+        let fetched =
+            NewSubtitleFile::fetched(file, "pt-PT", "opensubtitles", "1", "/m/one.pt-PT.srt");
+        record_file(db.pool(), &fetched).await.unwrap();
+
+        // A different file, the same language: `.mt.srt` never sits beside a
+        // real subtitle.
+        let translated = NewSubtitleFile::translated(file, "pt-PT", "deepl", "/m/one.pt-PT.mt.srt");
+        let refused = record_file(db.pool(), &translated).await.unwrap_err();
+        assert!(
+            matches!(&refused, sqlx::Error::Database(error) if error.is_unique_violation()),
+            "{refused:?}"
+        );
+
+        // Another language on the same file, and the same language on another
+        // file, are both untouched by it.
+        record_file(
+            db.pool(),
+            &NewSubtitleFile::fetched(file, "en", "opensubtitles", "2", "/m/one.en.srt"),
+        )
+        .await
+        .unwrap();
+        let other = media_file(db.pool(), "/m/two.mkv", "2026-01-01T00:00:00.000Z").await;
+        record_file(
+            db.pool(),
+            &NewSubtitleFile::fetched(other, "pt-PT", "opensubtitles", "3", "/m/two.pt-PT.srt"),
+        )
+        .await
+        .unwrap();
+
+        assert_eq!(files_for(db.pool(), file).await.unwrap().len(), 2);
+    }
+
+    /// The embedded key is untouched by the sidecar one: a track inside the
+    /// video is not a file, and a plain and a forced track for one language
+    /// legitimately coexist (#222).
+    #[tokio::test]
+    async fn embedded_tracks_still_share_a_language() {
+        let (db, _dir) = database().await;
+        let file = media_file(db.pool(), "/m/tracks.mkv", "2026-01-01T00:00:00.000Z").await;
+
+        record_file(db.pool(), &NewSubtitleFile::embedded(file, "en"))
+            .await
+            .unwrap();
+        record_file(db.pool(), &NewSubtitleFile::embedded(file, "en").forced())
+            .await
+            .unwrap();
+        record_file(db.pool(), &NewSubtitleFile::embedded(file, "en").sdh())
+            .await
+            .unwrap();
+        // And a sidecar for that same language still fits beside all three.
+        record_file(
+            db.pool(),
+            &NewSubtitleFile::extracted(file, "en", "/m/tracks.en.srt"),
+        )
+        .await
+        .unwrap();
+
+        assert_eq!(files_for(db.pool(), file).await.unwrap().len(), 4);
+    }
+
+    #[tokio::test]
+    async fn sidecar_for_answers_with_the_file_not_the_track() {
+        let (db, _dir) = database().await;
+        let file = media_file(db.pool(), "/m/ask.mkv", "2026-01-01T00:00:00.000Z").await;
+
+        record_file(db.pool(), &NewSubtitleFile::embedded(file, "en"))
+            .await
+            .unwrap();
+        assert_eq!(
+            sidecar_for(db.pool(), file, "en").await.unwrap(),
+            None,
+            "an embedded track is not a sidecar"
+        );
+
+        record_file(
+            db.pool(),
+            &NewSubtitleFile::extracted(file, "en", "/m/ask.en.srt"),
+        )
+        .await
+        .unwrap();
+        assert_eq!(
+            sidecar_for(db.pool(), file, "en").await.unwrap().as_deref(),
+            Some("/m/ask.en.srt")
+        );
+        assert_eq!(sidecar_for(db.pool(), file, "pt-PT").await.unwrap(), None);
+    }
+
+    /// A sidecar row written straight in, so a test can build the duplicate
+    /// state the schema now forbids.
+    async fn raw_sidecar(
+        pool: &SqlitePool,
+        file: i64,
+        origin: &str,
+        language: &str,
+        path: &str,
+        created: &str,
+    ) {
+        sqlx::query(
+            "INSERT INTO subtitle_files
+                 (media_file_id, language, origin, provider, engine, path, created_at)
+             VALUES (?, ?, ?, ?, ?, ?, ?)",
+        )
+        .bind(file)
+        .bind(language)
+        .bind(origin)
+        .bind((origin == "provider").then_some("opensubtitles"))
+        .bind((origin == "translated").then_some("deepl"))
+        .bind(path)
+        .bind(created)
+        .execute(pool)
+        .await
+        .unwrap();
+    }
+
+    /// A database written before the constraint may hold duplicates from a
+    /// manual grab that beat the API's path check. Migration `0029` resolves
+    /// them instead of failing: a real subtitle beats a machine translation,
+    /// and of two of the same kind the newest wins (#222).
+    #[tokio::test]
+    #[allow(clippy::too_many_lines)]
+    async fn the_migration_resolves_duplicates_it_finds() {
+        let (db, _dir) = database().await;
+        let file = media_file(db.pool(), "/m/old.mkv", "2026-01-01T00:00:00.000Z").await;
+
+        // Stand the database back up as it was before `0029`.
+        sqlx::query("DROP INDEX subtitle_files_one_sidecar")
+            .execute(db.pool())
+            .await
+            .unwrap();
+
+        let pool = db.pool();
+        // Newer, but machine made: the real subtitle is the one to keep.
+        raw_sidecar(
+            pool,
+            file,
+            "provider",
+            "pt-PT",
+            "/m/old.pt-PT.srt",
+            "2026-01-01T00:00:00.000Z",
+        )
+        .await;
+        raw_sidecar(
+            pool,
+            file,
+            "translated",
+            "pt-PT",
+            "/m/old.pt-PT.mt.srt",
+            "2026-02-01T00:00:00.000Z",
+        )
+        .await;
+        // Two of the same kind: the newest wins.
+        raw_sidecar(
+            pool,
+            file,
+            "provider",
+            "en",
+            "/m/old.en.srt",
+            "2026-01-01T00:00:00.000Z",
+        )
+        .await;
+        raw_sidecar(
+            pool,
+            file,
+            "provider",
+            "en",
+            "/m/old.en.2.srt",
+            "2026-03-01T00:00:00.000Z",
+        )
+        .await;
+        // An embedded pair for one language is not a duplicate.
+        sqlx::query(
+            "INSERT INTO subtitle_files (media_file_id, language, origin, forced)
+             VALUES (?, 'en', 'embedded', 0), (?, 'en', 'embedded', 1)",
+        )
+        .bind(file)
+        .bind(file)
+        .execute(db.pool())
+        .await
+        .unwrap();
+
+        let migration = crate::MIGRATOR
+            .iter()
+            .find(|migration| migration.version == 29)
+            .expect("migration 0029");
+        sqlx::raw_sql(migration.sql.clone())
+            .execute(db.pool())
+            .await
+            .unwrap();
+
+        let kept: Vec<_> = files_for(db.pool(), file)
+            .await
+            .unwrap()
+            .into_iter()
+            .filter_map(|row| row.path)
+            .collect();
+        assert_eq!(kept, vec!["/m/old.en.2.srt", "/m/old.pt-PT.srt"]);
+        assert_eq!(
+            files_for(db.pool(), file)
+                .await
+                .unwrap()
+                .iter()
+                .filter(|row| row.origin == SubtitleOrigin::Embedded)
+                .count(),
+            2,
+            "embedded rows are not sidecars and are left alone"
+        );
+    }
+
     #[tokio::test]
     async fn the_schema_ties_every_optional_column_to_the_origin() {
         let (db, _dir) = database().await;