I ended up to solve this by deleting content of tombstones tables straight in SQL, as pointed in this thread. But I wish there were a way to do that without putting hands on database.
EDIT: problem is still unsolved… I had to revert steps above because, without tombstones data, I cannot delete records from WorldCat. But then setSpec identifiers are duplicated again… Is this a bug in OMP? Any ideas to fix this?