fix: check before adding a required field to the list

[umutuzgur]

Check if a field is already added to the required list

[robmcguinness]

@umutuzgur can you add small test?

[umutuzgur]

Hey @robmcguinness, could you guide me where to add it? I'm not familiar with the repo, I was just trying to fix an issue we have since we upgraded the dep to version 16

[robmcguinness]

Inspect https://github.com/hapi-swagger/hapi-swagger/blob/master/test/unit/property-test.js#L175

[umutuzgur]

Thanks. I still don't really know why we have the field twice in the required list but we have it after I upgraded from v14 to v16. I will add a test tomorrow after I figure out why

[umutuzgur]

@robmcguinness done!