By mistake, I have send second time 0.09923 BTC
to bitcoin address:
15qMMPEa4M26XX4KqXxrvD5yjNwfETg3Hx via transaction id:
I did the transaction twice. How to refund it ?
You've signed over the bitcoins to your trading partner. If you have any contact details, you can try to explain the situation to them and request that they return the unintended payment.
As there are no chargebacks or other ways to cancel a payment in Bitcoin, you're at the mercy of their goodwill or would need to pursue legal action. Basically, the situation is the same as if you had paid in cash and could prove that you had paid twice.