The sentences that contain an example 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 contested usage here is the preposition "to" at the end of the sentence, which some grammarians argue against.)
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"The chair for the chair of the board of directors is at the head of the board table." (The contested usage here is the use of "chair" as both a noun to refer to a person and as a term for the position, which can cause confusion.)
These two sentences involve contested grammatical structures or definitions that can be debated among language users.