Job gave me a raise before my start date

I recently got hired for a job, and I got a letter in the mail saying that the board of directors approved a ~4% raise for my position for the "new fiscal year." My hire date was 2 months before I actually started, and the raise was after I got hired. However, the letter was dated before my start date, so I think there may have been a mistake. Should I tell HR? I don't want to get fired or get in trouble for not saying anything... but I also want to keep the raise. Seems like a strange mistake though, but I did work at this place as a contractor before which may have caused the mistake.

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