A little late, but thanks to all who responded.........

I was finally able to isolate the actions which caused the TABLEUPDATE failure.

During processing the program would DELETE some records in the cursor. It would do some more processing and then issue a RECALL ALL before doing the TABLEUPDATE.
(Please don't ask why....it's not relevant to this discussion)

Apparantly the RECALLed records are seen as requiring an INSERT. But because the record was only DELETED in VFP it remained in MySQL. When TABLEUPDATE tried to do the INSERT it generated a DUPLICATE KEY error.

I was able to circumvent this by trapping for the failure, examining the error message and, when appropriate, issuing a DELETE on the backend, and then resuming the processing.

Again.....Thanks to all for your responses.

.........Rich