title: TypeError: __init__() got multiple values for argument 'instance'
body: <p>I was tring to keep ckeditor and markdown at same time in a edit/(add new post) in blog web backend, so i modifid <code>__init__</code>,after that,i can still open edit page .but,once I click save button, this error came out:<code>TypeError: __init__() got multiple values for argument 'instance'</code></p>
  
  <pre><code>Traceback (most recent call last):
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\core\handlers\exception.py", line 34, in inner
      response = get_response(request)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\core\handlers\base.py", line 126, in _get_response
      response = self.process_exception_by_middleware(e, request)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\core\handlers\base.py", line 124, in _get_response
      response = wrapped_callback(request, *callback_args, **callback_kwargs)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\contrib\admin\options.py", line 604, in wrapper
      return self.admin_site.admin_view(view)(*args, **kwargs)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\utils\decorators.py", line 142, in _wrapped_view
      response = view_func(request, *args, **kwargs)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\views\decorators\cache.py", line 44, in _wrapped_view_func
      response = view_func(request, *args, **kwargs)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\contrib\admin\sites.py", line 223, in inner
      return view(request, *args, **kwargs)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\contrib\admin\options.py", line 1640, in change_view
      return self.changeform_view(request, object_id, form_url, extra_context)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\utils\decorators.py", line 45, in _wrapper
      return bound_method(*args, **kwargs)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\utils\decorators.py", line 142, in _wrapped_view
      response = view_func(request, *args, **kwargs)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\contrib\admin\options.py", line 1525, in changeform_view
      return self._changeform_view(request, object_id, form_url, extra_context)
    File "E:\python_works\project\django\typeidea-env\lib\site-packages\django\contrib\admin\options.py", line 1556, in _changeform_view
      form = ModelForm(request.POST, request.FILES, instance=obj)
  TypeError: __init__() got multiple values for argument 'instance'
  </code></pre>
  
  <p>this is the part where once i delete, error won't come out:</p>
  
  <h1>blog/adminforms.py</h1>
  
  <pre><code>class PostAdmin(forms.ModelForm):
      ....
      def __init__(self, instance=None, initial=None, **kwargs):
          initial = initial or {}
          if instance:
              if instance.is_md:
                  initial['content_md'] = instance.content
              else:
                  initial['content_ck'] = instance.content
          super().__init__(instance=instance, initial=initial, **kwargs)
      ....
  </code></pre>