First of all, "repeat" doesn't actually have a prefix. So, you aren't "peating" something again.
It's a word derived from French:
repeat
late 14c., from O.Fr. repeter "say or do again, get back, demand the return of" (13c.), from L. repetere "do or say again, attack again," from re- "again" + petere "go toward, seek, demand, attack" (see petition). Specific meaning "to take a course of education over again" is recorded from 1945, Amer.Eng. Related: Repeated; repeating. The noun is first recorded 1550s.
It has a prefix in Latin, but not in English.
As you can see, "repeat" can mean 'to do or say again', so "repeat" can be used to mean to 'resay'.
Interestingly, etymonline.com and dictionary.com both don't give records of 'resay'.