How do I create an advanced route with multiple slugs?
Brad nails the problem in his comment:


  The error is because you've got two named subpatterns with the same name (&lt;slug&gt;)


And the solution is simple... Change the name(s) of your subpattern(s).

The term slug is just a convention of Craft. It doesn't matter what the names are, they can be anything useful to you:

(?P&lt;domain&gt;[^/]+)/(?P&lt;category&gt;[^/]+)/feed


Whatever values match those named patterns are automatically passed into the Twig template as "magic" variables. So your Twig template can now make use of the domain and category values.