Deepcopy on ErrorDict shows unexpected duplicates

[Troy Grosfield]

Using python 3.4, django 1.7, you'll see the following error when deepcopying ErrorDicts:

$ python
Python 3.4.1 (default, Aug 24 2014, 21:32:40)
[GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from copy import deepcopy
>>> from django.forms.utils import ErrorList
>>> from django.forms.utils import ErrorDict
>>>
>>>
>>> ed = ErrorDict()
>>> ed['__all__'] = ErrorList(['first thing'])
>>> ed
{'__all__': ['first thing']}
>>> ed_copy = deepcopy(ed)
>>> ed_copy
{'__all__': ['first thing', 'first thing']}   # <-- what happened here? why is "first thing" listed twice?
>>> len(ed['__all__']) == len(ed_copy['__all__'])
False

Why did "first thing" get copied twice?

[Tim Graham]

Seems to be a regression from 1.6.

[Tim Graham]

Bisected to 2fd7fc134cf0c0685ceac22fd858509aa43f819f. Regression test attached.

[Loic Bistuer]

PR ​https://github.com/django/django/pull/3316.

[Loic Bistuer <loic.bistuer@…>]

In ec2fd02bb3f9f7e732de1566e81f84637e29f161:

Fixed #23594 -- Fixed deepcopy on ErrorList.

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

[Loic Bistuer <loic.bistuer@…>]

In 082abce81e48fe9397cce096e13d5199c99adfe9:

[1.7.x] Fixed #23594 -- Fixed deepcopy on ErrorList.

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

Backport of ec2fd02bb3 from master