Record merge implemented in SQL fails with a unique violation (23505) as soon as the secondary record has side data. Unique values (email, domain) live in a side table rebuilt by an AFTER UPDATE trigger on the record (delete all its rows, re-insert from props). If the merge removes the old records' unique rows by hand, then moves associations or visits, other triggers (recomputed activity properties) update the OLD record, the sync trigger re-inserts its email, which is already taken by the new record, and the whole merge transaction aborts.