This one is tough. I have to say, I spent way too much time on Reddit. But I more often saw people complaining about reposts on content I'd never seen before, than I saw the same content over and over again.
But when I did see a repost, it seemed to pop up over and over again. So it is annoying.
It's sort of a constant of the internet. Some things change and disappear forever, like guestbooks on your personal webpage. And some things are forever, like complaining about somebody posting something you've seen before.