(#6749) detect duplicate aliases in a single option statement.

This ensures that an option declaration that shadows itself is found, and
reported to the user, rather than silently eating one of the two.

This could have actually lost, for example, the distinction between an
argument-requiring and an argument-missing variant of the same thing.

Reviewed-By: Pieter van de Bruggen <pieter@puppetlabs.com>