If the girl who says that is with you, and straight, then point out something about the girl she mentioned that's a flaw. "Yeah, she's good looking and all but she's got cankles" or whatever. Or if you can't find a flaw in her looks simply say "Yeah but she looks like a real b***h!" That always works. If she's with you or hanging out with you, then she's saying that in a fishing for compliments kinda way. If she does it, you should make it a point to compliment your girl more often. Unless you don't want a girl thats emotionally high maintenance, then say something instead like "Yeah she is. Hang on a second" and go ask the other girl for her number. 
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I'm giving it another shot. We will see.
My forum is still there and everyone is welcome to come join as well. There is a private women only subforum there if anyone is interested. Also, there is no CAPTCHA.

The link to the forum is
http://www.rightplanet.proboards.com