Prevent duplicate generated migration field

[fbuys]

It is possible to generate a resource with duplicate fields.
I think it makes sense to prevent this from happening and rather warn that a duplicate field is present?

Right now you can generate a resource with two titles like this:
lucky gen.resource.browser Project title:String title:String

I cannot think of a scenario where you would want two duplicate fields, I don't think this will work in Postgres either.

I am happy to work on a PR if this feature sounds reasonable?

[jwoertink]

That's a great catch! We should definitely prevent that.