Can't transit through the same city twice in the same direction unless there is no other option... but that shouldn't be a problem, as you can always go through ZRH, MUC or other European hubs instead of FRA twice.

The other thing to keep in mind and for maximizing the journey is that in addition to your point of turn-around and stopover points, of which you are allowed one in each direction, you can have a stopover in other cities along the way as long as they are less than 24 hours. This is really simple in cities like frankfurt, where if you arrive at 7pm for example there are many options to depart within 24 hours, and the stop-over should get you into the FCT.