User story.
As a user, I want to know if the tags defined in my AsyncAPI document (in any place) have a unique names.

Is your feature request related to a problem?
The following AsyncAPI document is invalid and Spectral should throw errors due to non-uniqueness of tags.*.name.

asyncapi: '2.3.0'
...

tags:
  - name: someTag
    ...
  - name: someTag
    ...

Additional context
Part of #2100