We have experienced something that looks a little like a session leak in earlier version of 12. If this behaviour is being observed, I would suggest looking into an upgrade to a later 12.20 version.
Otherwise, multiple sessions can be caused by:
More tha none browser / client used (which was confirmed not to be the case here)
Document not yet opened and still loading, but user attempts to open it again.
Server overloaded in the backend and unable to clear out sessions.
Issues with file access to the .pgo files storing license data
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One additional clarification to Sonja's comment, there is a difference of behavior when using Session CALs versus Named/Document CALs in that if you have Session CALs and the user does NOT have a Named/Doc CAL assigned, and they use multiple machines or browsers as noted, there will be multiple sessions created by the user as well, and this is as expected behavior in that case. Doing an update to latest SR in the 12 track you are running would be a good first step as Sonja noted, as there were issues in the earlier SRs of some of the 12 tracks and the engineers did make changes to improve things.
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