I'm tiling a shower. I put 6mil vapor barrier behind the cement board but weant to be sure the application is entirely waterproof. I'd like to cover the cement board with RedGard prior to tiling. I've read a few posts on the internet saying this is a bad idea because it will "trap" water in the cement board! I realize that the Red Gard may be overkill, but would it be problem to use it over cement bpard that has a vapor barrier behind it?

You do not need the redgard with the vapor barrier, it can trap moisture and be a problem. You'll be entirely water proof once the tile is up without it.

Like Amricca said... can't have 2 vapor barriers here.

You will be fine with the one BEHIND the cement board!

Good luck!

MARK