[BUG] Argparse does not handle argument de-duplication

[Jacobfaib]

To reproduce:

int main(int argc, char *argv[])
{
  argparse::ArgumentParser program("program_name");

  program.add_argument("--foo")
         .default_value(1)  

  try {
    program.parse_args(argc, argv);
  } catch (const std::exception& err) {
    std::cerr << err.what() << std::endl;
    std::cerr << program;
    std::exit(1);
  }
}

$ ./example --foo 1 --foo 1
Duplicate argument --foo
...

The parser should simply accept the last value of the argument, to match the behavior of the majority of other argument parsers

[wanjiadenghuo111]

亲，来信已经收到！祝你快乐！

[Jacobfaib]

related to #397