Importer should detect when a directory contains multiple copies of the same album

if an album is present multiple times in a directory, beets does seem to announce the duplicate tracks as "unmatched tracks". Instead it should check if the unmatched track titles match the album in question and report it and give the option to silently skip.

Start with the importer entry point that reports duplicate tracks as unmatched. Trace how unmatched track titles are compared with the album in the directory, then verify that duplicate albums are reported and that the user can choose to skip them silently.

Written by the indexing model from the issue text.