While making the payment, I applied for the discount code but my transaction was unsuccessful. When I try again, it tells me the code is already used. What do I do?

That sounds upsetting. Don't lose the spirit!

If the transaction failed you can reuse the voucher starting with '7' post 30 minutes as it is blocked for safety measures.

In case it is a voucher starting with '8', voila! Reuse it right away. 

Please stand by till the mentioned time. If the issue persist we're all ears to guide you.

If it's a Filmy pass coupon code you can re-use it post 20 mins.