When a rule generates a file with exactly the same name as a previous file, and moves it to the destination folder, it overwrites the existing file.
I, personally, would prefer a behaviour similar to that of MacOSX where it creates a new file with a (1) or a (2) or (x) after the filename.
Often, when the system can't parse the date, it just starts applying the filedate with the < match > variable and you end up with files being overwritten.
Cheers,
Jordan