Usage: $chkdup(string1,string2,...,stringN)

Returns 0 if there are no duplicates, N if there are N duplicates.

Code:
alias chkdup {
  scon -r var % $+ a = $* $!!cr ,%b = $!addtok(%b,%a,13)
  return $calc($0 - $numtok(%b,13))
}

Strings may not contain $cr or an escaped version of $cr.
It takes $null's into consideration, so $chkdup(,,) returns 2.