This is related to connect #424800. As described in this blog post:
We can now delete entries from the MRU list by using the delete key (or Fn+Delete if, like me, you're using SSMS inside VMWare Fusion).
However, we may have four entries pointing at the same server, and we want to keep the one with the right credentials and/or auth mode, but we can't tell which one it is, since the list only exposes server name. The workaround is to delete all of the duplicates and create a new one from scratch, but this is less than optimal and - with a quick fix - could me made much more direct and predictable.