one_of runtime violation has same error message as a duplicate argument

[cmannett85]

$ ./example_basic_cat_cpp20 --max-line-length=10 --skip-line --line-suffix=foo
terminate called after throwing an instance of 'arg_router::parse_exception'
  what():  Argument has already been set: --line-suffix=foo
Aborted (core dumped)

The argument hasn't already been set but because a one_of acts as a single node under a mode as soon as one value is set, mode will detect when another is attempted to be set and throw.