You will pay €5,60 in advance at the beginning of each month. The total cost of your journeys is billed a month afterwards, together with the costs of the subscription for the new month. Your bill is correct. You do not pay twice for the subscription.
Another matter though: According to the dates of your bills you may have two personal chipcards as you receive two bills each month. Likely both with the same kind of subscription. You paid €5,60 at the 18th of October and at the first of each month as well. This is useless normally, one subscription is enough.Do you pay for another person and you do not know? Is your own name on both bills?
It may be wise to stop one of them. Do have a look at Mijn NS to see your bills and you will know.
Looks like you have another (unused) OV-chipkaart with NS Flex Dal Voordeel.
See the € 5,60 invoices for the card number. You probly replaced (lost) or renewed the card the wrong way, and ordered another subscription on a new card.
The best way is to cancel one of them. As far as I can remember it is a policy of NS to give three months back by Customer Service. Probably it is leniency, not a right. I cannot find where I read it and @Robert B cannot find it as well. It is € 16,80 in that case.