diff --git a/scripts/merge_altkey_tags.py b/scripts/merge_altkey_tags.py
index e8867a9..8401986 100644
--- a/scripts/merge_altkey_tags.py
+++ b/scripts/merge_altkey_tags.py
@@ -85,23 +85,29 @@ def main() -> None:
             s.rollback()
             return
 
+        # UWAGA: NIE robimy UPDATE tag_id (kolizja PK gdy scena ma DWA warianty tego
+        # samego klastra → oba UPDATE na keep = duplikat (scene_id, keep)). Zamiast tego
+        # DODAJEMY membership kanonicznego (DISTINCT + ON CONFLICT), a wiersze dropów
+        # sprzątnie CASCADE przy DELETE FROM tags. Kolizyjno-odporne.
+        # EXISTS-guard na keep: gdyby kanoniczny zniknął między budową _dup_map a INSERT
+        # (współbieżny ingest/cleanup), pomijamy zamiast walić FK violation (całość rollback).
         r1 = s.execute(text("""
-            UPDATE scene_tags st SET tag_id = m.keep_id
-            FROM _dup_map m
-            WHERE st.tag_id = m.drop_id
-              AND NOT EXISTS (SELECT 1 FROM scene_tags k
-                              WHERE k.scene_id = st.scene_id AND k.tag_id = m.keep_id)
+            INSERT INTO scene_tags (scene_id, tag_id)
+            SELECT DISTINCT st.scene_id, m.keep_id
+            FROM scene_tags st JOIN _dup_map m ON st.tag_id = m.drop_id
+            WHERE EXISTS (SELECT 1 FROM tags kt WHERE kt.id = m.keep_id)
+            ON CONFLICT (scene_id, tag_id) DO NOTHING
         """))
-        log.info("scene_tags migrated: %d", r1.rowcount)
+        log.info("scene_tags keep-memberships inserted: %d", r1.rowcount)
 
         r2 = s.execute(text("""
-            UPDATE movie_tags mt SET tag_id = m.keep_id
-            FROM _dup_map m
-            WHERE mt.tag_id = m.drop_id
-              AND NOT EXISTS (SELECT 1 FROM movie_tags k
-                              WHERE k.movie_id = mt.movie_id AND k.tag_id = m.keep_id)
+            INSERT INTO movie_tags (movie_id, tag_id)
+            SELECT DISTINCT mt.movie_id, m.keep_id
+            FROM movie_tags mt JOIN _dup_map m ON mt.tag_id = m.drop_id
+            WHERE EXISTS (SELECT 1 FROM tags kt WHERE kt.id = m.keep_id)
+            ON CONFLICT (movie_id, tag_id) DO NOTHING
         """))
-        log.info("movie_tags migrated: %d", r2.rowcount)
+        log.info("movie_tags keep-memberships inserted: %d", r2.rowcount)
 
         # blacklisted_tags PK = (device_id, tag_id) → przenieś ban z dropa na kanoniczny per device.
         r3 = s.execute(text("""