We have run into this in the past as well. If you take a look at the post below it should explain why this is happening and what you can do to fix it:
Quote from @aidacra in the above post:
the default option of Default will likely result in the occasional situation where userA submits report1 for print preview and userB will see it instead, as the Default setting uses only the Windows session ID of the user that submitted the request 
So, everyone should change their Work Station method to some value that will be guaranteed to always be unique for every user–for almost everyone, Machine Name + Domain User meets that requirement.