You could try making the Genre tags identical and rescanning, but sometimes the database includes separate entries for the same artist.
At the moment the most we can do is report it in this thread. The Roon guys (@joel in particular) work through these issues with AllMusic gradually in order to solve them.
In future (Roon 1.3), I understand the increased metadata editing will allow us to resolve the same artist/two entries equivalences without having to correct it upstream.