So here is what I did or You should do:
1. Find out where the nearest customer care center is, or You can get this job done at BSNL Tele shoppe. I was fortunate enough to have one at Greams Road, Chennai. Folks from Chennai can use the following link: http://chennai.bsnl.co.in/mktgchn/AllCSR.asp
2. In order to request for a replacement SIM card you require ID proof (Photo ID and Address proof) copies along with original for verification (At times they verify, better to carry it)
3. A request form should be filled, I took a pic of this form. You can download it using the link below.
4. Submit the completed form along with ID proofs and pay Rs. 20 (for a 2G sim).
5. You can pay Rs. 200 for a BSNL 3G SIM, but remember, 3G SIM card cannot be cut and used as a Micro SIM.
6. Once you pay the amount, a new SIM and receipt will be provided to you across the counter.
7. It takes 1 hour for the sim to get activated (I was instructed NOT to put the sim in dual sim phone till it gets activated - I don't know why, but just follow :D )
Alright guys, good luck to get your duplicate SIM cards!
*SIM is the abbreviation of Subscriber Identity Module.