I received an additional tax refund check in the mail for an unexpected amount. I had already received my correct tax refund via direct deposit. Is this check legitimate?

Possibly, but the IRS is suggesting you do not cash it until you get a letter explaining why you got the check.

"Not Entitled to Refund Received

If you receive a refund to which you're not entitled, or for an amount that's more than you expected, don't cash the check until you receive a notice that explains the difference; then follow the instructions on the notice. For a direct deposit that was greater than expected, review the notice to determine if the difference was correct, and follow the instructions on the notice. For information about returning an erroneous refund, see Topic No. 161."https://www.irs.gov/taxtopics/tc152