Amazon commingles[1] inventory. So if there are N vendors selling an item, including Amazon.com, all N inventories just get mixed together at the warehouses. So if some M of those N are counterfeit, there's no way to know.
"As an example, if I sell Duracell C batteries on Amazon through their “Shipping Fulfilled by Amazon” — which I must do to receive Prime shipping designation — I need to send my batteries to an Amazon warehouse. After receiving my delivery, they will count the number of batteries, then slide the whole stock into a generic shelf labeled “Duracell C Batteries.” Any purchaser receives a Duracell C battery from that box, and thus the actual seller is unknown."
"As an example, if I sell Duracell C batteries on Amazon through their “Shipping Fulfilled by Amazon” — which I must do to receive Prime shipping designation — I need to send my batteries to an Amazon warehouse. After receiving my delivery, they will count the number of batteries, then slide the whole stock into a generic shelf labeled “Duracell C Batteries.” Any purchaser receives a Duracell C battery from that box, and thus the actual seller is unknown."
1: https://thetriplehelix.medium.com/your-amazon-products-could...