[This looks like a bug report against PIP because Tarek told me distutils2 would be responsible for this kind of thing and that there was an open ticket for it.  However, I can't find any such ticket so I'm posting it here]

Not only does pip not check for conflicts as noted in http://bitbucket.org/ianb/pip/src/tip/pip/req.py#cl-928, it doesn't consider any requirements on a package other than the first. So if I have

A requires B and C
B requires D<=1.1
C requires D<=0.9

installing A will give us D==1.1 rather than D==0.9

Once responsibility for this functionality is sorted out, we may be able to close this ticket or http://bitbucket.org/ianb/pip/issue/119