Say no, if they left me for another girl once whats to say they won't do it again. i would tell myself i can do better and keep my dignity by showing them i've moved on (even if i haven't) cause it was them that messed up...then honestly i would either try to move on with another relationship or...just stay single for a while and concentrate on other things :-) if this is your situation....trust me chick, you can do better

A leopard doesn't change its spots!

If you had to ask this question with such a simple answer, it seems you still have feelings for him. I am about 75% sure the reason why he came back to you now is because he got dumped by his new girlfriend. If that is indeed true, what will you do? Do you just want to be backup girlfriend at his convenience until he find another? Think about it carefully.

I'd probably already be dating someone else. So it doesn't matter.

I suppose it depends on the situation and on the relationship -- what you both put into it, what you both want out of it. But you should never allow people to disrespect you.