title: Django: namespace isn't unique
body: <p>In Django 1, I used to have the following URL mappings:</p>
  
  <pre><code>...
  url(r'^main/', include('main.urls', namespace='main')),
  url(r'.*', include('main.urls'))
  </code></pre>
  
  <p>The <code>r'.*'</code> mapping is always at the last line to take care of all kinds of URLs not mapped.</p>
  
  <p>In Django 2, the following mappings are used instead:</p>
  
  <pre><code>path('main/', include('main.urls', namespace='main')),
  re_path('.*', include('main.urls')),
  </code></pre>
  
  <p>Although it also works, yet Django complains:</p>
  
  <pre><code>?: (urls.W005) URL namespace 'main' isn't unique. You may not be able to reverse all URLs in this namespace
  </code></pre>
  
  <p>Giving the second mapping another namespace is not working. Any solutions?</p>