Renew NS

i have 50 days left to my subscription for NS Deluxe, if i renew it now with a credit card, will this 50 remaining days will be added to the 1 year and if i renew it via credit card, can it be possible that they accidentally charge up twice the amount because im seing people complaining that they got charged twice the amount for only one year. I also see people that got charged even when they dont use NS?

Thanks for any help in advance.

G