I have been finding on occasion that the first field on a form is being duplicated. I usually delete the second field and all is ok for a while but it does happen again.
I use iCloud to sync forms between iPhone, iPad and Mac.
Does the deletion of the duplicate field get removed from your other devices during a sync?
Try closing and re-opening the document on each device to see if that solves the problem.
Yes removing on one syncs to other devices. I’ll have to wait for a reoccurrence to see if closing and opening document has an effect.