Supplying the same argument twice doesn't cause error

These arguments:
```
args::ValueFlag<std::string>  a1  (parser, "a1", "a1", {'d', "arg1"});
args::ValueFlag<std::string>  a2  (parser, "a2", "a2", {'d', "arg2"});
```
aren't valid since both are activated with ```-d```, but ```args``` doesn't fail.
