How do you detect and fix a fan-out in Alteryx?

Note the left record count before the join. Afterwards, L plus J should equal it. If J is larger than expected, the right input has duplicate keys and each left row has matched several times.

The fix is to put a Summarize on the right input first, grouping by the join key so there is exactly one row per key.

It matters because nothing errors. The workflow runs, the numbers look plausible, and every measure downstream is inflated.