installing something to a virtualenv when it's already in site-packages

[nbv4]

I have ipython installed via apt. I can go to the command line and
type 'ipython' and it will work. If I try to install ipython to a
virtualenv, I get this:

$ pip install -E env/ ipython
Requirement already satisfied: ipython in /usr/share/pyshared
Installing collected packages: ipython
Successfully installed ipython

I want ipython in both site-packages as well as in my virtualenv. This
is bad because when I activate the virtualenv, site-packages
disappears and ipython is not available. A work around is to uninstall
ipython from apt, install to the virtualenv, then reinstall in apt. Is
there a better way?