The message here:

        if (!br.atEof()) {
            severe() << "Unique index cursor seeing multiple records for key "
                     << redact(curr(kWantKey)->key);
            fassertFailed(28608);
        }

..could be enhanced to supply the index name as well. The index name is a member of WiredTigerIndex, and the IndexCursorBase object where the message is generated has a reference to the WiredTigerIndex instance.