As discussed on forum.

Currently all artists have "Show all release groups instead, or show various artists release groups." at the bottom of their "Overview" page as well as "Show Various Artist releases instead." on their Releases page. Since these links in many cases are "no-ops" this leads to Google (and presumably other search engines) as recording the resulting URL as a duplicate page (since it’ll be exactly identical).

A solution to this would be to either straight up remove the links if they wouldn’t actually show anything, or (probably better) replace with a (non-linked) text like "There are no non-official or various artists release groups by the artist."

(Are the "?va=1" links included in the sitemap?)