Fixed #23594 -- Fixed deepcopy on ErrorList.

Thanks Troy Grosfield for the report and Tim Graham for the tests.

django/forms/utils.py

@@ -138,6 +138,15 @@ class ErrorList(UserList, list):
             return list(error)[0]
         return force_text(error)
 
+    def __reduce_ex__(self, *args, **kwargs):
+        # The `list` reduce function returns an iterator as the fourth element
+        # that is normally used for repopulating. Since we only inherit from
+        # `list` for `isinstance` backward compatibility (Refs #17413) we
+        # nullify this iterator as it would otherwise result in duplicate
+        # entries. (Refs #23594)
+        info = super(UserList, self).__reduce_ex__(*args, **kwargs)
+        return info[:3] + (None, None)
+
 
 # Utilities for time zone support in DateTimeField et al.