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, suggesting it should be "to whom this letter is addressed.")
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The chair for the chair of the board of directors is at the head of the board table. (The word "chair" can be contested in its use as a noun to refer to a person in position of authority, as some may prefer "chairperson" for gender neutrality.)
Both instances involve language usage that can be debated or have alternative conventions.