RBL Paisabazaar duet card is good for 1% statement credit (you will get cash back in 7-10 days after transaction, none of cards will provide cash back for unbilled transaction except HDFC smartbuy). As this card is LTF, no need to worry. You can keep this card for RBL bank offers.
If you want to have more RBL cards, then it's better to disclose your credit card portfolio for better solution.