If a user logs in into our DNN portal ( 5.6.3 Professional), he gets his own open session. So far, so good. At the same time, from another machine, this user (or somebody else) can login into the same session ( using the same credentials  - tried it, it works). 

Is there a way to prevent that?  Maybe by means of iis configuration, if not possible in DNN itself? What we need is to have only one login available per user/per session (in other words: if a user with the same credentials tries to login from another machine the first session from this user should be killed automatically).

The basic premise is not correct.

You can't login to the same session. 

You can login more than once with the same credentials, but the two logins do not share session information.

Why do you want to prevent this?  What are you trying to stop?

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Best wishes,
- Richard
Agile Development Consultant, Practitioner, and Trainer
www.dynamisys.co.uk

I'm still not clear why this you want to do this.  The concept of 'wins' does have meaning for me.

If I login at machine A I can do some work.  If I then move to machine B I can login and do more work.  If I go back to machine A I can carry on with still more work.  The state of the system is as I last left it from machine B.

What does 'wins' mean in this context?

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Best wishes,
- Richard
Agile Development Consultant, Practitioner, and Trainer
www.dynamisys.co.uk