The criterion behind it
4.1.1 Parsing
This criterion was originally adopted to address problems that assistive technology had directly parsing HTML. Assistive technology no longer has any need to directly parse HTML. Consequently, these problems either no longer exist or are addressed by other criteria. This criterion no longer has utility and is removed.
Level A · EN 301 549 § 9.4.1.1
How it is reported
A11ySignal reports this at serious impact. Every failing element comes with its CSS selector and the markup that failed, so you can go straight to the line rather than hunting for it. Deque publishes the engine-level detail:
https://dequeuniversity.com/rules/axe/4.13/duplicate-id-active ↗
Check implemented by axe-core 4.13.0, © Deque Systems, MPL-2.0.
Other checks for 4.1.1
minorduplicate-idid attribute value must be unique