Use PDM Workgroup, it is free with the Pro and Premium versions and it will let you know if the number is already in the system, plus it will track your revisions too.
If you can't implement a PDM, which would address the issue as noted above, try a filing structure that would prevent duplicate file names. For example, try something like
If there are too many files within the "SMS" directory, use sub-directories like "...\SMS\1" for files "SMS-1*", and "...\SMS-2" for files "SMS_2*", but be consistent. Either always or never use these sub-directories.
If the file tracking is manual (you don't have a PDM) then the file location should also be logical, and the file name should imply the correct filing location.
My opinion, I guess.