message: Fixed issue found in
  persist-doctests can't be in RUSTDOCFLAGS more than once. This change prevents that from occurring.
diff: @@ -247,8 +247,10 @@ fn setup_environment(config: &Config) {
  common_opts, DOCTEST_FOLDER
  );
  if let Ok(vtemp) = env::var(rustdoc) {
  + if !vtemp.contains("--persist-doctests") {
  value.push_str(vtemp.as_ref());
  }
  + }
  env::set_var(rustdoc, value);
  }
  