<p>is there any way of making sure that, one user is logged in only once?</p>

<p>I would like to avoid two different persons logging into the system with the same login/password.</p>

<p>I guess I could do it myself by checking in the django_session table before logging in the user, but I rather prefer using the framework, if there is already such functionality.</p>

<p>Cheers,</p>

<p>Thanks for the responses!</p>

Logged in twice is ambiguous over HTTP.  There's no "disconnecting" signal that's sent.  You can frustrate people if you're not careful.
If I shut down my browser and drop the cookies -- accidentally -- I might be prevented from logging in again.
How would the server know it was me trying to re-login vs. me trying to login twice?
You can try things like checking the IP address.  And what if the accidental disconnect was my router crashing, releasing my DHCP lease?  Now I'm trying to re-login, but I have a new address and no established cookie.  I'm not trying to create a second session, I'm  just trying to get back on after my current session got disconnected.
the point is that there's no well-established rule for "single session" that can be installed in a framework. You have to make up a rule appropriate to your application and figure out how to enforce it.