I'm assuming that there's a spacing issue when you used copy/paste, because there's no space before /id
SubmitToSharePointList /submit=WebServiceSubmit /mapping=Qmapping/id=my:ID
should be
SubmitToSharePointList /submit=WebServiceSubmit /mapping=Qmapping /id=my:ID
Is that correct?
We would need to make sure that:
1. The relative xpath my:ID is correct (matches your schema)
2. That the ID is correctly stored in that field. If the field is blank, a new item will be created.
Can you display the ID field in the canvas temporarily and let us know whether it is being populated with an ID after submitting the list items? Thanks.