I feel like every time there is a discussion about Amazon on the Internet, this point is brought up.
I spent about $9,000 on Amazon across 139 orders in 2018. I buy a lot on Amazon. I've never had a problem with receiving knockoff products. I don't understand how this can be such a prevalent issue when I've not run into it, given how much I use Amazon.
The only thing I can think is that, if I'm buying something expensive, I'll strictly only purchase items "Sold by: Amazon.com Services, Inc" or alternatively, I'll use a marketplace seller if the seller is the manufacturer of the item I'm buying.