core: add notice about duplicate keys

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

authentik/core/api/utils.py

@@ -14,7 +14,7 @@ def is_dict(value: Any):
     """Ensure a value is a dictionary, useful for JSONFields"""
     if isinstance(value, dict):
         return
-    raise ValidationError("Value must be a dictionary.")
+    raise ValidationError("Value must be a dictionary, and not have any duplicate keys.")
 
 
 class PassiveSerializer(Serializer):