Playing games over and over is something I do all the time. The vast majority of games I own I have done so more than once. Knowing the plot or puzzles or whatnot doesn't deter me from playing again. If the game is heavily dependent on one of those, then either it does it well enough that I won't mind seeing it again, or is just not a game I would like in the first place.
Probably the best example for me is Paper Mario. I seriously try to beat this game at least once every single year. Its not like the plot is amazing, or the puzzles tough or anything, but the game-play is just enjoyable every single time through, and no matter how many times I read the dialogue, it still makes me laugh. I don't even need to put up special challenges to have fun, and I often find myself 100 percenting it just out of habit because I like talking to everyone an everything each time I play it. While the difficulty might not be there as it was when I first played it as a little kid, the enjoyment is as great, if not greater than it ever was.