One final question - anything wrong with having *2* vapor barriers?

Just got back from HD, and all they have for 2x4 walls was Kraft-faced
batts. Can I put those up and then the foam over it? OC makes an R-15
2x4 batt now, so at least I got a little more than R13. Id like to use
the foam if I can, but not if its bad to have dual vapor barriers
(assuming the Tuff-R acts as a vapor barrier)

Thanks!
-Chris