Using VSS Plugin 1.6.1 and Eclipse 2.2 (though I've experienced this bug throughout the life of VSS Plugin) I can always multiply check out a file someone else has checked out even if the VSS project is not set to allow multiple check outs.
Steps to reproduce:
1. Both people get the latest copy of a VSS project
2. Person A checks out a file for editing
3. Person B checks out the same file for editing prior to doing a refresh or update state but subsequent to step 2.
VSS Plugin will allow both A and B to have the file checked out at the same time. If A then checks in his changes, after check-in the file will display as checked-out by B. B's check-in will overwrite A's changes without notification.
However, if after step 2 Person B does a refresh or update state, then the file in question will be shown as checked out and prevent B from checking out the file.
Thanks,
Brion Swanson