diff --git a/wagtail/wagtailimages/migrations/0004_make_focal_point_key_not_nullable.py b/wagtail/wagtailimages/migrations/0004_make_focal_point_key_not_nullable.py
index 8ec529e31..0abd20c59 100644
--- a/wagtail/wagtailimages/migrations/0004_make_focal_point_key_not_nullable.py
+++ b/wagtail/wagtailimages/migrations/0004_make_focal_point_key_not_nullable.py
@@ -5,41 +5,26 @@ from django.db import models, migrations
 
 
 def remove_duplicate_renditions(apps, schema_editor):
-    if schema_editor.connection.vendor == 'mysql':
-        schema_editor.execute("""
-            DELETE FROM `wagtailimages_rendition` WHERE CONCAT(image_id, '-', filter_id) IN (
-                SELECT CONCAT(image_id, '-', filter_id)
-                FROM (SELECT * FROM `wagtailimages_rendition`) AS x
-                WHERE `focal_point_key` IS NULL
-                GROUP BY image_id, filter_id
-                HAVING COUNT(*) > 1
-            ) AND `focal_point_key` IS NULL
-        """)
-    elif schema_editor.connection.vendor == 'microsoft':
-        schema_editor.execute("""
-            DELETE FROM [wagtailimages_rendition] WHERE CAST(image_id AS VARCHAR(MAX)) + '-' +
-            CAST(filter_id AS VARCHAR(MAX)) IN (
-                SELECT CAST(image_id AS VARCHAR(MAX)) + '-' + CAST(filter_id AS VARCHAR(MAX))
-                FROM [wagtailimages_rendition] WHERE focal_point_key IS NULL GROUP BY image_id,
-                filter_id HAVING COUNT(*) > 1
-            ) AND focal_point_key IS NULL
-        """)
-        schema_editor.execute("""
-            DECLARE @constraint_name VARCHAR(MAX)
-            SELECT @constraint_name = CONSTRAINT_NAME FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
-            WHERE TABLE_NAME='wagtailimages_rendition' AND CONSTRAINT_TYPE='UNIQUE'
-            EXEC('ALTER TABLE [wagtailimages_rendition] DROP CONSTRAINT ' + @constraint_name)
-            ALTER TABLE [wagtailimages_rendition] ALTER COLUMN [focal_point_key] NVARCHAR(255) NOT NULL
-            EXEC('ALTER TABLE [wagtailimages_rendition] ADD CONSTRAINT ' + @constraint_name +
-            ' UNIQUE NONCLUSTERED (image_id, filter_id, focal_point_key)')
-        """)
-    else:
-        schema_editor.execute("""
-            DELETE FROM wagtailimages_rendition WHERE image_id || '-' || filter_id IN (
-                SELECT image_id || '-' || filter_id FROM wagtailimages_rendition
-                WHERE focal_point_key IS NULL GROUP BY image_id, filter_id HAVING COUNT(*) > 1
-            ) AND focal_point_key IS NULL
-        """)
+    Rendition = apps.get_model('wagtailimages.Rendition')
+
+    # Find all filter_id / image_id pairings that appear multiple times in the renditions table
+    # with focal_point_key = NULL
+    duplicates = (
+        Rendition.objects.filter(focal_point_key__isnull=True).
+        values('image_id', 'filter_id').
+        annotate(count_id=models.Count('id'), min_id=models.Min('id')).
+        filter(count_id__gt=1)
+    )
+
+    # Delete all occurrences of those pairings, except for the one with the lowest ID
+    for duplicate in duplicates:
+        Rendition.objects.filter(
+            focal_point_key__isnull=True,
+            image=duplicate['image_id'],
+            filter=duplicate['filter_id']
+        ).exclude(
+            id=duplicate['min_id']
+        ).delete()
 
 
 def reverse_remove_duplicate_renditions(*args, **kwargs):