In case of a slow sync or any addition of a supposedly new item it is necessary to find out if an identical or similar item already exists in the database. Usually this is what UIDs are for. But unfortunately, we cannot rely on a UID since an entry most likely will have two different UIDs when originating from two different clients. Or maybe the same entry has been entered in two different address books which were not able to sync until now. So we have to find identical items. But how similar is identical? Or otherwise, how do we know which items can be safely merged?