Rollup merge of #147683 - jdonszelmann:less-duplicate-checking, r=JonathanBrouwer

only check duplicates on old/unparsed attributes

r? ``@JonathanBrouwer``

This was effectively already what we were doing, but this was implicit because `.name()` etc were just returning None when dealing with a parsed attribute.... this makes it explicit

Author: matthiaskrgr