COUCHDB-968 we discovered and fixed a bug that could introduce duplicate documents in a database. Under certain circumstances these duplicate documents would introduce duplicate entries in view indexes.
We can detect duplicates in the view group by looking for duplicate entries in the docid => view key btree. We can try to repair the view indexes by removing the extra keys from the views, or we can just report that duplicates exist in the view group and direct the user to rebuild that group. We might incorporate this code into the view compactor, or we might release it as a separate standalone tool.
I think my personal preference is to include any repair functionality in the view compactor. A separate standalone tool that simply reports the presence of duplicates would also be a nice thing to have.