Try this

; /rename.doubles "path" filemask

; ex.1 /rename.doubles "test" test*.txt
; ^^ only if "path" is a subfolder of mircdir

; ex.2 /rename.doubles "C:\test files" *

; Won't work if the DIR has double spaces
Code:
alias rename.doubles {
  tokenize 34 $1-
  window -h @@
  var %i = 1,%a = $findfile($1,$mid($2,2),0,echo @@ $1 $longfn($1)).shortfn
  while $line(@@,%i) {
    tokenize 32 $ifmatch
    if !$isfile($2-) { .rename $1 $+(",$2-,") }
    inc %i
  }
  window -c @@
}