diff --git a/alembic/versions/0022_source_per_artist_platform.py b/alembic/versions/0022_source_per_artist_platform.py
index 2b2853e..f85b96a 100644
--- a/alembic/versions/0022_source_per_artist_platform.py
+++ b/alembic/versions/0022_source_per_artist_platform.py
@@ -83,55 +83,70 @@ def upgrade() -> None:
         if not other_ids:
             continue
 
-        # STEP 2: PRE-merge colliding Posts BEFORE the bulk reparent.
-        # Find pairs (keep, drop) where:
-        #   keep  = a Post under the canonical source
-        #   drop  = a Post under one of the non-canonical sources whose
-        #           external_post_id equals keep.external_post_id
-        # If we let the bulk UPDATE try to repoint drop onto canonical,
-        # uq_post_source_external_id fires row-by-row and aborts the
-        # whole migration.
-        colliding = conn.execute(
+        # STEP 2: PRE-merge ALL Posts with duplicate external_post_id
+        # across the entire (canonical + others) group, BEFORE the bulk
+        # reparent. Two cases must both be handled:
+        #   (A) canonical has Post X with epid=N; an "other" source has
+        #       Post Y with epid=N → after bulk UPDATE, (canonical, N)
+        #       collides with itself.
+        #   (B) two different "other" sources each have a Post with
+        #       epid=N; canonical has none → after bulk UPDATE, both
+        #       are repointed to (canonical, N) and the second collides.
+        # The earlier version of this migration only handled (A); the
+        # operator's deploy 2026-05-26 tripped (B) at line 139.
+        # Fix: group ALL Posts in the (artist, platform) by epid; for
+        # any group with count>1, pick the keep (prefer one already
+        # under canonical; else lowest id) and merge the rest into it.
+        all_posts = conn.execute(
             text("""
-                SELECT keep.id AS keep_id, drop_.id AS drop_id
-                FROM post AS keep
-                JOIN post AS drop_
-                  ON drop_.external_post_id = keep.external_post_id
-                 AND drop_.id != keep.id
-                WHERE keep.source_id = :canonical
-                  AND drop_.source_id = ANY(:others)
+                SELECT external_post_id, id, source_id
+                FROM post
+                WHERE source_id = :canonical OR source_id = ANY(:others)
+                ORDER BY external_post_id, id
             """),
             {"canonical": canonical_id, "others": other_ids},
         ).fetchall()
-        for keep_id, drop_id in colliding:
-            conn.execute(
-                text("""
-                    UPDATE image_provenance SET post_id = :keep
-                    WHERE post_id = :drop_
-                """),
-                {"keep": keep_id, "drop_": drop_id},
-            )
-            conn.execute(
-                text("""
-                    UPDATE image_record SET primary_post_id = :keep
-                    WHERE primary_post_id = :drop_
-                """),
-                {"keep": keep_id, "drop_": drop_id},
-            )
-            # Repointed provenance may now collide on
-            # uq_image_provenance_image_post (alembic 0021). Dedupe:
-            # keep min(id) per (image_record_id, post_id).
-            conn.execute(text("""
-                DELETE FROM image_provenance ip1
-                USING image_provenance ip2
-                WHERE ip1.image_record_id = ip2.image_record_id
-                  AND ip1.post_id = ip2.post_id
-                  AND ip1.id > ip2.id
-            """))
-            conn.execute(
-                text("DELETE FROM post WHERE id = :drop_"),
-                {"drop_": drop_id},
-            )
+        by_epid: dict = {}
+        for epid, post_id, src_id in all_posts:
+            by_epid.setdefault(epid, []).append((post_id, src_id))
+        for epid, posts in by_epid.items():
+            if len(posts) <= 1:
+                continue
+            # Prefer a Post already under canonical as the keep.
+            canonical_posts = [p for p in posts if p[1] == canonical_id]
+            if canonical_posts:
+                keep_id = canonical_posts[0][0]
+            else:
+                keep_id = posts[0][0]  # already sorted by id ASC
+            drop_ids = [p[0] for p in posts if p[0] != keep_id]
+            for drop_id in drop_ids:
+                conn.execute(
+                    text("""
+                        UPDATE image_provenance SET post_id = :keep
+                        WHERE post_id = :drop_
+                    """),
+                    {"keep": keep_id, "drop_": drop_id},
+                )
+                conn.execute(
+                    text("""
+                        UPDATE image_record SET primary_post_id = :keep
+                        WHERE primary_post_id = :drop_
+                    """),
+                    {"keep": keep_id, "drop_": drop_id},
+                )
+                # Repointed provenance may now collide on
+                # uq_image_provenance_image_post (alembic 0021). Dedupe.
+                conn.execute(text("""
+                    DELETE FROM image_provenance ip1
+                    USING image_provenance ip2
+                    WHERE ip1.image_record_id = ip2.image_record_id
+                      AND ip1.post_id = ip2.post_id
+                      AND ip1.id > ip2.id
+                """))
+                conn.execute(
+                    text("DELETE FROM post WHERE id = :drop_"),
+                    {"drop_": drop_id},
+                )
 
         # STEP 3: Bulk reparent the remaining Posts off the other
         # Sources. After step 2, no collisions on