There was some confusing behavior with &optional and locals, so this
should get rid of that by making it an error. However, there is a case
where function parameters are still allowed to have &optional - this is
because there are checks for &default in parameters as well.
- Replaced "tag" with "attribute" in the error message since the former
is not exactly the same concept/meaning and also not user-facing
terminology
* origin/topic/timw/174-duplicate-attributes:
Allow duplicate attributes in full redefs
Short-circuit checking of whether attr exists
Expanded check for other tag types, fixed btest to cover more tags
GH-174: Add warning for duplicate attributes
