which version are you on?
To narrow down the problem you could download the Governance Dashboard and look at what it tells you about the sessions.
This might give you a hint of how the server and the document are used by those users.
But besides that it looks like the session is not terminated correctly.
Take a look at this post (Re: Access Point Timeout Settings) and check the corresponding configuration in the QMC.
For your training needs: Check training.qlik.com for a Server/Publisher-Training near you. This is a great jump start to administering QV server and it will give you the opportunity to ask any open questions as well.
Thanks Tobias, that helps me with some of the diagnostics...
I'm on version 11.00.11414.0409.10.
Looking at the Governance Dashboard, all of the sessions started within 5-10 seconds of each other. That is really strange. In which case, timeout settings would not have helped...at least until the timeout was reached. I know there is a concurrent session limit option, but I don't think I can limit it at the user/document level, e.g. User X can only have Y sessions of Z document open at one time.
So, I'm still trying to figure how a document can be opened 18 times so quickly, and how that can be prevented.