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WCAG: 4.1.1: ID attribute values must be unique
The value assigned to an ID attribute must be unique to prevent the second instance from being overlooked by assistive technology. Put another way; ID attributes may not be used more than once in the same document to differentiate each element from another.
The ID attribute uniquely identifies elements on a page. It does not make sense to duplicate an ID.
Duplicate ID's can break the accessibility of labels for forms, table header cells, etc., by the second instance being skipped by screen readers, or by client-side scripts. They are common markup validation errors that can eliminate possible sources of accessibility problems, when not breaking accessibility.
Those experienced with client-side scripting know that when you re-use an id, typically the only one that gets acted upon by the scripting is the first instance of the use of that ID. Similarly, assistive technologies may, when referencing an id, only reference the first one accurately.