Twice now I've been able to do the same prestige missions over again.
It appears that not only are prestige missions doable once per day, but also once per rank.
I'm fairly certain that that isn't the intent of the system.
However I haven't been able to recreate this 100%, and may have to do with server timing.
I was able to do this from Squire to Soldier.
From Soldier to Knight I was unable to do this. I attempted this very close to the Prestige mission daily change (12am PST), so that may have been a factor.
From Knight to Knight Elite I was able to recreate the redoable Prestige missions. (sometime between 3pm-4pm)
Cheers, and how about some phat lute if this helped ;)
P.S.
Sorry for breaking forum rules is this is indeed an exploit.