Technically it is leagal, it can be deferred as many times as you wish as long as there is one good flight leg between. It is all up to the operator and the CAA that signed off on their GMM. Most companies have some sort of "chronic dis." program, usually 3 repeats should draw extra attention but usually will not have verbage to ground the aircraft so long as said APU was operating at the time it was signed off.
Alot of the time a "ghost" of a problem, say a relay that sticks occasionally or a valve that sticks sometimes just needs enough time to "break hard" after all can't be expected to fix what is not broken. Option B is to shotgun parts at it $$$$$$$$.