If one tries to add twice the same flag to an ArgumentParser, one gets a helpful exception:

    from argparse import ArgumentParser
    p = ArgumentParser()
    p.add_argument("--foo")
    p.add_argument("--foo")

results in

    argparse.ArgumentError: argument --foo: conflicting option string: --foo

However, adding twice the same subparser raises no exception:

    from argparse import ArgumentParser
    p = ArgumentParser()
    sp = p.add_subparsers()
    sp.add_parser("foo")
    sp.add_parser("foo")

even though the two subparsers shadow one another in the same way as two identical flags.