If you have more than 100 duplicates, the script will go into an infinite loop. That is, it'll try:

prefix.msg
prefix 01.msg
prefix 02.msg
...
prefix 98.msg
prefix 99.msg
prefix 00.msg
prefix 01.msg <== Oops! Already tried this one
prefix 02.msg <== Ditto.
...


You ignore Murphy at your peril.

Thanks for pointing that out. From what I can tell, in any given folder, I don't have more than 99 dups. But I'll make a note of that in case I have to re-use this script for such a case.