I filed my taxes in Jan 2021 and requested the 2nd stimlus check since i hadn't received it and it was included in the refund. I just got my stimulus check in the mail. Do I cash the check?

Yes, you can go ahead and cash the check.

TurboTax has established a procedure for taxpayers with this same Recovery Rebate Credit situation. 

Please see the TurboTax Help article I've already claimed the Recovery Rebate Credit. How can I review and update my 2020 tax return? for guidance.

The IRS requires you to report all relevant stimulus information when you’re filing your return.