What version are you running?
RBTools 1.3.4, reviewboard 1.6.1
Currently post-review doesn't handle merges well. If you have merged with the upstream repository during your last development cycle, and then post-review you'll get a diff that includes changes that are already in upstream. This is not ideal. The review attached resolves this issue, see the review for a more detailed explanation on the cases that will now be taken into account.
What operating system are you using? What browser?
Tested with Ubuntu Natty (11.04) (Python 2.7, mercurial 1.7.5).
I have enhanced rbtools myself and uploaded the diff to:
http://reviews.reviewboard.org/r/2628/