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WCAG: 4.1.1: ID attribute value must be unique
The value assigned to active ID attributes on focusable elements must be unique to prevent the second instance from being overlooked by assistive technology. Put another way, active ID attributes may not be used more than once on focusable elements in the same document; focusable active elements require unique IDs for assistive technology to distinguish one element from another.
The ID attribute uniquely identifies focusable elements on a page. It does not make sense to duplicate an active ID.
Duplicate active ID values break the accessibility of focusable elements including labels for forms, table header cells, etc., Screen readers and client-side scripts will skip any duplication other than the first instance. Validating HTML files help prevent and eliminate possible sources of accessibility problems, when not breaking accessibility.
Those experienced with client-side scripting know that when you re-use an active ID, typically the only one that gets acted upon by the scripting is the first instance of the use of that active ID. Similarly, assistive technologies may, when referencing an active ID, only reference the first one accurately.