The sentences that contain examples of contested usage are:
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"The postman needs to know who this letter is addressed to so it can be delivered." - The phrase "who this letter is addressed to" can be contested because some grammar purists argue that "to whom this letter is addressed" is more grammatically correct.
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"The chair for the chair of the board of directors is at the head of the board table." - The term "chair" can be contested as it has undergone a shift in usage, where some people still prefer "chairman" or "chairwoman," while others advocate for "chair" as a gender-neutral term.
(Note: If additional context defines "contested usage" differently, please clarify.)