Is this a romantic relationship, or just a friendship? I'd say in romance, unless you've got whatever it is that Kendall has, second chances are just asking for trouble.But if we're talking friendship here, I have to disagree with gnu. It may have taken the friend 3 years of counselling and such to both get over whatever was wrong that made them hurt you in the first place, then get the nerve up to even approach you to ask for forgiveness.
Oh yes, there's the other thing. Have they actually APOLOGIZED for hurting you? Or merely asked for a second chance? If just the latter, I'd say no. If they can't admit they have done you wrong and ask for forgiveness, they're not in a place (IMHO) where they can be trusted again.
But if they have asked for forgiveness, and really want to try again, then I'd go with the approach BEK gave: If you want a relationship go into it with your eyes open to the potential. Set your boundaries. Test that person's resolve to be the kind of friend that you need. You have the right for your own self protection.
Go much more slowly than you would with someone you're meeting new. Any close relationship involves letting down our defences, little by little, over a period of time, as the person "earns" it by treating us well after the last defence-slackening. I'm not saying this well, but I hope you get the drift. With someone who has betrayed you in the past, I would move much more slowly in the letting-down-the-defences department, and always be ready to slam them back in place again if it seems necessary.
Contrary to BEK, you may get hurt. Being a friend at all involves the risk of being hurt; there is no other way to be a friend. But it would be a long time before I trusted a second-chancer with anything that would really hurt me. Slow and cautious.
And if they blow it again, for me it would be over, endsville, done-for, finis, das Ziel, khattam-shud.
JMHO.
Alex