Better args handling for Dedup

[ReallyLiri]

the original code has a bug, the cast check is flipped:

t, err := arg.(int)
if err ...

should be

t, ok := arg.(int)
if !ok ...

But anyway changed to a more go styled code

[paul-mannino]

Thanks for the PR & bug fix. Would you mind adding a couple tests that exercise these optional args? Coverage seems a little lacking for those code paths currently.