I gave 21 k cash to a person, and paid me back through online payment, but now he is claiming that I don't have paid any money to him what should I do?

Dear Sir,

                A lawyer can send a notice stating:

The repayment transaction proves no dues are left.

His demand amounts to extortion and cheating.

Any further threats will lead to FIR.

This usually stops 99% of such people.

If you have proof or payment or witness before whom you had made the payment  to substantiate the payment then refute the allegation but if you have neither witness nor any proof then return the money to friend so as to avoid complications.