If you have a prompt in the query to enter the TID you will get it twice:
- first time to populate the textbox on the form
- second time to run the insert


So no unexpected behaviour here. Instead of prompting in the query again (which I assume is the same value as the one entered in the combo that triggers the NotInList event), why not make reference to it in the query: Forms!frmYourSimpleForm!cboYourCombo(maybe .Text if the record is not yet saved as it probably isn't)

Cheers,
Vlad