I have a hard time believing it is a problem with the number generator, it is driven by SQL...that is a pretty tried and true technology.
Is it possible you have several templates using the same counter? Could one of those counters have the setting "Disable standard serial number generation" in step 4 turned on? Maybe it is a template not used very often causing the problem?
I've seen this kind of problem twice, each in a different vault.
Case 1: Suddenly for no reason (?!) the number generator generates an old-existing number every time any user does Save As from within SolidWorks for Assembly/Part files.
To "fix" the problem I had to do "Copy Paste" of files in the vault a few times to generate some more "old" serial numbers then suddenly the number generator starts generating correctly.
Case 2: It still happens once in a while (mostly seen with MS Office files), when a user copies a file out of the vault, makes some modification and saves it back to the vault under a different name, the number generator sometimes does not generate a new value (even though in the data card, field "Document Number" is set to use "Serial Number").
No templates are involved in any case.
It's hard for me to believe too!!
No, they have all happened using the same template and they do not have the Disable Serial Number Generation check box checked and this template is used every day. We do have two templates using the same serial number but all of the duplication has happened on one template because the other is not used very often.
These excel files are being created inside of the vault, there is no saving outside of the vault or save as with a different name.
We have been able to look into the SQL server while the duplication is hapening and there are no errors or warnings. The duplication will last for maybe 5 files (-09, -092, -093, -094) and then it will return to normal.
We have it happen sometimes when a user does a save in solidworks but decides to cancel. It's not reproducible though.