At the end of the day, it is
Amazon that activates the subscription. BSNL may send the activation request in such cases. Amazon's activation routine may not be buggy enough to activate the same number with the same Amazon account again. We may try getting another FTTH account with LCO, try activating it for
Amazon Prime, then disconnect the old account. At least, my LCO wouldn't mind, as he is very close to me. But, if I were given the chance to write the software that activates the subscription I wouldn't activate more than one
free subscription (via BSNL, Airtel or whatever).
Basically my point is that it is easy to catch and neglect such duplicate free subscription request using software, regardless of plan change, number change or ISP change.
I have high respect for developers behind Amazon. I don't think they would allow any such requests.