title: Django Login Form Returning false on is_valid() if username already exists
body: <p>I have a login form I created from Django's <code>User</code> model:</p>
  
  <p><strong>forms.py</strong>:</p>
  
  <pre><code>class LoginForm(ModelForm):
      class Meta:
          model = User
          fields = ['username', 'password']
          widgets = {
              'username': forms.TextInput(attrs={'placeholder': '@username'}),
              'password': forms.PasswordInput(attrs={'placeholder': 'Password'})
          }
  </code></pre>
  
  <p><strong>views.py</strong>:</p>
  
  <pre><code>    reg = LoginForm(request.POST or None)
      if reg.is_valid():
          return HttpResponse('Success: valid form')
      else:
          return HttpResponse('Error: invalid form')
  </code></pre>
  
  <p>Now if I try login in with a <em>username</em> that is not registered it returns <strong>Success: valid form</strong> but if it is a 
  username that already exists, it says <strong>Error: invalid form</strong>.</p>
  
  <p>I tried doing this in command line and below is what I get (I first tried it with a username that is register, <strong>yax</strong>):</p>
  
  <pre><code>In [2]: data = {'username':'yax', 'password':'wrong_password'}
  In [3]: form = LoginForm(data)
  
  In [4]: form.is_valid()
  Out[5]: False
  In [6]: form.errors
  Out[7]: {'username': [u'A user with that username already exists.']}
  
  In [8]: data = {'username':'wrong_name', 'password':'wrong_password'}
  In [9]: form = LoginForm(data)
  
  In [10]: form.is_valid()
  Out[11]: True
  In [12]: form.errors
  Out[13]: {}
  </code></pre>
  
  <p>Would be glad I can know why I am getting this error and how do I solve it. </p>