Yes, the IRS knows that your spouse is deceased based on Social Security records/info. However you can keep your spouse's stimulus check if the death occurred after January 1, 2020.
Joint filers with a deceased spouse: For payments made to joint filers with a deceased spouse who died before January 1, 2020, return the decedent’s portion of the payment. This amount will be $1,200 unless your adjusted gross income exceeded $150,000.
https://www.irs.gov/newsroom/questions-and-answers-about-the-first-economic-impact-payment-topic-a-e...
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