> but even today most don't have the concept of "I've already seen this".

Maybe I'm missing something obvious, but the way that normally works, is that you go to the last post you remember reading, then continue from there. So start at last page, go back until you remember what you're coming across, continue reading from there.

Probably only works for small/medium-sized forums though, the biggest forum I frequent only have ~20-40K users active and most (popular) threads die after 300-400 posts.