This exact same thing happened to me this summer.
Normally, I'm a one chance type of person - you burn me once, you don't get another chance to burn me.
I was dating a girl this summer, she really had alot going for her. She had just gotten out of a lengthy relationship... we starting seeing eachother, and I thought I finally found someone that was generous and caring like me.
Well, the ex-boyfriend kept pestering her - saying he would change his ways.
She never told him about me, and guess what? She never told me that she was still seeing him on the side as well.
This goes on all summer, and then she tells me she needs "space". I point blank ask if it is the other guy, and she says "I still have feelings for him, but it's not a relationship that I want to continue, and I know it's one that isn't good for me."
A week later, she's engaged to this guy. Ouch.
Moral of this story: It's great to be a nice guy, and to give people the benefit of the doubt... but don't be blind. She may genuinely like you, but she is also stringing you along until she is convinced that the ex really has changed his ways. Then you are going to get dropped like a hot potato. The irony is, a tiger never changes it's stripes- this guy is going to hurt her again. And there is nothing you can do about it.
My advice: Tell her that she has to choose, and not to contact you until she does. If you are serious about this girl being a good catch, tell her that. Then tell her that you can't continue this relationship until she is 100% committed to you, and only you. If she can't do this, the the reality is, she's not the one for you.
Good luck - keep us updated on how this turns out.