message: alive: give error on register redefinition
diff: @@ -857,10 +857,10 @@ static void parse_fn(Function &f) {
  while (true) {
  switch (auto t = *tokenizer) {
  case REGISTER: {
  - // FIXME: add error checking for repeated names
  string name(yylval.str);
  auto i = parse_instr(name);
  - identifiers.emplace(move(name), i.get());
  + if (!identifiers.emplace(move(name), i.get()).second)
  + error("Duplicated assignment to " + string(yylval.str));
  bb->addInstr(move(i));
  break;
  }
  