Simon Franck and Anna were married prior to February 1576/7. It is almost certain that the Jacob, son of Simon Franck, who was christened on 17 February 1577/8 was the Jacob Duchmann who married Catharina Schmidt (among others) in July 1606. This is your line, barring any infidelity.
First names were common in Bas-Rhin, but not universal. Remember the confusion about the two sets of Jacob Duchmanns and Johann/Hans Jacob Duchmanns.
There is no doubt that both Hanß Jacob Duchmann (and wife Apollonia Gams) and Jacob Duchmann (and most likely his first wife, during the hiatus in the Taufbuch in the early 1600s) had daughters named Agnes. Both young ladies survived to adulthood and both married and their marriage records, and baptismal sponsor entries, are conclusive.