Handle duplicate urls in urlconf

[jcushman]

Django resolves urls in order from top to bottom, and only uses the first matching URL found. This can happen for example when overriding one view in an existing app:

    path('accounts/login/', user_views.LoginView.as_view(), name='login'),
    path('accounts/', include('django.contrib.auth.urls')),  # logout, password change, password reset

drf-yasg doesn't follow Django's behavior, and instead uses the last view for a given path+method. This means duplicate URLs are documented incorrectly in the resulting schema.

This PR adds a test to show the correct behavior, and a filter to OpenAPISchemaGenerator.get_api_endpoints to ensure that each path is only included once.

[axnsan12]

Awesome, thanks for the catch!

[jcushman]

Thanks for the test fix and quick merge!