diff --git a/litellm/proxy/proxy_server.py b/litellm/proxy/proxy_server.py
index a80a277dc48..82f147b3f9d 100644
--- a/litellm/proxy/proxy_server.py
+++ b/litellm/proxy/proxy_server.py
@@ -12637,7 +12637,13 @@ async def update_config(  # noqa: PLR0915
             existing_cb = existing.get("success_callback")
             if isinstance(incoming_cb, list):
                 if isinstance(existing_cb, list):
-                    merged["success_callback"] = list(set(existing_cb + incoming_cb))
+                    # Normalize the existing list too — a row written by a
+                    # different code path may still hold mixed-case names,
+                    # which would otherwise dedup-miss against the lowercase
+                    # incoming entries.
+                    merged["success_callback"] = list(
+                        set(normalize_callback_names(existing_cb) + incoming_cb)
+                    )
                 else:
                     merged["success_callback"] = list(set(incoming_cb))
 
diff --git a/tests/test_litellm/proxy/management_endpoints/test_update_config_endpoint.py b/tests/test_litellm/proxy/management_endpoints/test_update_config_endpoint.py
index a425e55708c..2797b13b4e6 100644
--- a/tests/test_litellm/proxy/management_endpoints/test_update_config_endpoint.py
+++ b/tests/test_litellm/proxy/management_endpoints/test_update_config_endpoint.py
@@ -208,6 +208,30 @@ def test_success_callback_unioned_with_existing(admin_auth, patched_proxy):
     assert set(stored) == {"langfuse", "prometheus"}
 
 
+def test_success_callback_dedups_against_mixed_case_existing(admin_auth, patched_proxy):
+    """
+    Regression: a litellm_settings row written by an older code path (or by
+    direct DB edit) may still hold mixed-case callback names like
+    ["Langfuse"]. When the user submits ["langfuse"], the union must
+    normalize the existing entries too — otherwise the DB ends up with both
+    "Langfuse" and "langfuse" and delete_callback (lowercase lookup) cannot
+    find the original.
+    """
+    prisma = patched_proxy(
+        initial_rows={"litellm_settings": {"success_callback": ["Langfuse", "SQS"]}}
+    )
+
+    client = TestClient(app)
+    resp = client.post(
+        "/config/update",
+        json={"litellm_settings": {"success_callback": ["langfuse"]}},
+    )
+
+    assert resp.status_code == 200
+    stored = prisma.db.litellm_config.rows["litellm_settings"]["success_callback"]
+    assert set(stored) == {"langfuse", "sqs"}
+
+
 def test_success_callback_normalized_on_first_write(admin_auth, patched_proxy):
     """
     Regression: when no litellm_settings row exists yet, incoming mixed-case