If > 1 row on source side matches the same row on target side that means that we are forced update (or delete) the same row in target more than once as part of the same SQL statement. This should raise an error per SQL Spec
Section 14.2 under "General Rules" Item 6/Subitem a/Subitem 2/Subitem B
There is no sure way to do this via static analysis of the query.
Can we add something to ROJ operator to pay attention to ROW_ID of target side row and compare it with ROW_ID of target side of previous row output? If they are the same, that means > 1 source row matched.
Or perhaps just mark each row in the hash table that it matched. And if it matches again, throw an error.