duplicate-id-aria
IDs referenced by ARIA must be unique
Checks that any id value referenced by an ARIA ID-reference attribute (aria-labelledby, aria-describedby, aria-owns, aria-controls, aria-activedescendant, aria-flowto, aria-errormessage, aria-details) is unique in the document.
Applies to. Applies when the document contains at least one non-empty aria-labelledby, aria-describedby, aria-owns, aria-controls, aria-activedescendant, aria-flowto, aria-errormessage, or aria-details attribute (i.e. at least one ARIA ID reference exists to resolve).
Expectation. For every id value referenced by one of those attributes, exactly one element in the document carries that id. A duplicate does not break the reference: it resolves to the first element in tree order, so the name is still computed. Whether that element is the intended target depends on author intent, which markup does not carry, so the outcome is cantTell.
Examples
<span id="lbl">Name</span><input type="text" aria-labelledby="lbl">
The id `lbl` is unique, so the reference resolves unambiguously.
<span id="lbl">Name</span><span id="lbl">Duplicate</span><input type="text" aria-labelledby="lbl">
`aria-labelledby="lbl"` resolves to the first element with that id — whether that's the intended target is an authoring question the markup can't answer.