please include function appendToFile.

static void appendToTextFile_uniq(File f, S s) {
  if (empty(s)) ret;
  lock appendToFile_lock;
  S text = loadTextFile(f);
  if (!contains(lines(text), s))
    appendToFile(f, (empty(text) || endsWithNewLine(text) ? "" : "\n") + assureTrailingNewLineIfNempty(s));
}