Code:
on *:FILERCVD:*:{
  if ((%move) || (!$isfile($filename))) { return }
  if (!$isdir(Completed)) { mkdir Completed }
var %i
  while ($isfile(Completed $+ \ $+ $nopath($filename) $+ %i)) %i = $calc(%i + 1)
  .timer 1 0 .rename " $+ $filename $+ " " $+ Completed $+ \ $+ $nopath($filename) $+ %i $+ "
}


Very quick paste into remotes...
And, yeah, I know tossing on the %i to prevent the rename failure is pretty cheap. %move gets set and it doesn't use the feature.