My apologies. I seem to have tracked the problem down to the instance of the new form itself. I have created a new form with the same function and it is working as expected with no duplicates. The original form continues to produce duplicates. I can’t see why this is because the settings are the same but perhaps the initial form has been corrupted in some way.
I will see if I can find the reason and let you know why the problem manifested itself as a plugin conflict.