Check for duplicate attributes.
This adds some checks for duplicate attributes. In many cases, the duplicates were being ignored without error or warning. This adds several kinds of checks (see `AttributeDuplicates` enum).
The motivation here is to issue unused warnings with similar reasoning for any unused lint, and to error for cases where there are conflicts.
This also adds a check for empty attribute lists in a few attributes where this causes the attribute to be ignored.
Closes#55112.
f7c48297ce