Improvement proposal:
Let's have two items to be merged, which have two different sitelinks to a given MW site. One or both of these sitelinks may point to a redirect. If one redirect resolves to the other sitelink, it should be dropped automatically, otherwise the merge should fail.
Previous discussion: https://www.wikidata.org/wiki/MediaWiki_talk:Gadget-Merge.js#Resolve_redirected_sitelinks