message: Show error if `--target-dir` is provided more than once
diff: @@ -572,7 +572,12 @@ fn phase_cargo_miri(mut args: env::Args) {
  // Forward all arguments before `--` other than `--target-dir` and its value to Cargo.
  for arg in ArgSplitFlagValue::new(&mut args, "--target-dir") {
  match arg {
  - Ok(value) => target_dir = Some(value.into()),
  + Ok(value) => {
  + if target_dir.is_some() {
  + show_error(format!("`--target-dir` is provided more than once"));
  + }
  + target_dir = Some(value.into());
  + }
  Err(arg) => drop(cmd.arg(arg)),
  }
  }
  