Based on the issue description and the hints provided, I can analyze the problem and generate the fix. The issue is in Django's `django/contrib/auth/migrations/0011_update_proxy_permissions.py` migration file. Let me reconstruct the fix based on the detailed bug report.

The problem:
1. When a regular model is deleted and recreated as a proxy model with the same name, permissions already exist
2. The migration tries to update permissions for proxy models but fails with IntegrityError because the permission already exists with the content type it's trying to update to
3. The fix should handle the case where permissions already exist by reusing them instead of trying to create duplicates

Based on the issue (commit reference 181fb60159e54d442d3610f4afba6f066a6dac05 mentioned as the regression), the migration needs to be updated to handle the case where a permission with the target content_type_id and codename already exists.

Here's the patch:

--- a/django/contrib/auth/migrations/0011_update_proxy_permissions.py
+++ b/django/contrib/auth/migrations/0011_update_proxy_permissions.py
@@ -24,9 +24,24 @@ def update_proxy_model_permissions(apps, schema_editor, reverse=False):
             proxy_content_type = ContentType.objects.get_for_model(Model, for_concrete_model=False)
         old_content_type = cycled[concrete_content_type.pk]
         new_content_type = cycled[proxy_content_type.pk]
-        Permission.objects.filter(
+
+        # Get all permissions for this proxy model
+        permissions = Permission.objects.filter(
             permissions_query,
             content_type=old_content_type,
-        ).update(content_type=new_content_type)
+        )
+        for permission in permissions:
+            # Check if there's already a permission with the new content type
+            # and the same codename. If so, delete the old permission.
+            # Otherwise, update the content type.
+            try:
+                with transaction.atomic():
+                    permission.content_type = new_content_type
+                    permission.save()
+            except IntegrityError:
+                # Permission already exists with the new content type,
+                # delete the old one.
+                permission.delete()
 
 
 def revert_proxy_model_permissions(apps, schema_editor):
--- a/django/contrib/auth/migrations/0011_update_proxy_permissions.py
+++ b/django/contrib/auth/migrations/0011_update_proxy_permissions.py
@@ -1,3 +1,5 @@
+from django.db import IntegrityError, transaction
+
 from django.db import migrations
 from django.db.models import Q