OCaml Arg module triple output

open Arg

let main () =
  let cmds = [( "-a", Arg.String (fun str -> ()), "do stuff") ] in
  Arg.parse cmds (fun str -> ()) "UsageMsg"

let () = main ()


(* generates this output:

$ ocamlbuild my_prog.native; ./my_prog.native -a
Finished, 4 targets (0 cached) in 00:00:00.
./my_prog.native: option '-a' needs an argument.
UsageMsg
  -a do stuff
  -help  Display this list of options
  --help  Display this list of options
./my_prog.native: ./my_prog.native: option '-a' needs an argument.
UsageMsg
  -a do stuff
  -help  Display this list of options
  --help  Display this list of options
.
UsageMsg
  -a do stuff
  -help  Display this list of options
  --help  Display this list of options


*)