Rep has always been a sort of mystery, having said that it's nice to get it now and again, I think the reason behind not being able to rep the same person too soon is that a "mutual appreciation" scenario could develop and inside a few days you could give each other enough rep to have the wee green dots go off the end of the screen.
I don't know if there is an explanation as to how much you must spread around before giving it to the same person again, I have had the situation a couple of times where I thought someone deserved it again but alas no joy the little box appeared and said no no.
Even if a figure was mentioned as to how much had to be spread I doubt I would dish it out willy nilly just so that I could give it twice to someone.
Best just to accept it is the way it is and if you really need to acknowledge someone has made a good post or whatever you can always tell them in the thread if that makes sense.

Cheers

NCM