I have a series of Excel files on my PC that were created by another user. When I open some of the files Excel adds a 1 to the end of the file name. This then means that when I re-save it I end up with two files. Even if I remove the 1 as I save it it still ends up with two files. This does not happen with all of them, which is really confusing. I am using Excel 2003.
If the original files are in a folder to which you don't have write rights, or if they are read-only, then Excel will not be able to overwrite them, so will automatically save with a different name.