The system message is below:
This SqlTransaction has completed; it is no longer usable.

This is what I would expect since the SQL transaction being processed is creating a duplicate pk record.

I have discovered something significant about the sorting actions:

I position the List page on the 4th record (without sorting). When I click on the column header, the Ascending symbol appears and the record position doesn't move. When I click on the column header again, the Descending symbol appears but the displayed record changes to the 4th record from the BOTTOM of the list. The Properties page also shows details from the new record. However, it appears that the system is still referencing the foreign key for the previous record and tries to update the child records with the foreign key of the previous parent after updating the correct parent record.
If I click on the column header again (returning to Ascending sort), the list again changes to the 4th record from the beginning. I thought the record pointer should stay the same when sorting columns? At any rate, this "confusion" must be causing the update problems.