How do you prevent a Joiner from multiplying rows?

Row multiplication happens when the right table has duplicate values of the join key, so each left row matches several times.

Detect it by comparing the row count before and after. Fix it by adding a GroupBy on the right branch, grouped by the join key, so there is exactly one row per key.

It is worth turning on the Joiner's split output while developing — matched, left-unmatched and right-unmatched as separate tables. That shows you which keys failed rather than leaving you to notice a low total later.