It's permissible to hold multiple Lifetime Free Credit Cards from HDFC. Many members even report holding up to three such cards including "Infinia, DCB and another Co-Branded Card"
As long as you can provide proof that your card is LTF, HDFC won't charge you or close your account. However, to ensure complete safety, I recommend sending an email to HDFC and requesting a written confirmation that your card is LTF. This will come in handy in case of any future situations.
I hope this information is helpful, and I wish you the best of luck with your credit cards!