I believe that the OP is asking about the
UA "Round the World" Promo, which has the following routing restrictions.
Valid Routing: Travel must commence from and return to the same country and must be in a continuous global direction either Eastbound or Westbound. The direction of travel cannot be reversed once travel has commenced. Backtracking is not permitted and no city is to be transited more than once. Northbound and Southbound variations are not allowed. Travel must include only one Trans-Atlantic crossing and only one Trans-Pacific crossing. Travel may not be completed at a point beyond the point at which travel commenced.
In this case, the rules state that no city may be transited more than once, so connecting in FRA twice is likely prohibited. A possible solution might be IAD-FRA-PRG and then PRG-VIE-ATH.