This is a common refrain, but I would be amazed if it weren't backed up by data and I anecdotally fit into the mold. If I buy a product that I really like, then I'm likely to buy it again to give as a gift to friends. If I buy something like a a vacuum and I don't like it, then I'm likely to buy a different model that better fits my needs. It seems silly when you aren't interested in buying another one of whatever item you bought, but there are multiple legitimate reasons that people would want to.

Also, I bought my parents a Roomba because I liked mine, so that can happen too.

Ask the user if they liked the product and would likely gift it in the future and add the item to a "Gift ideas" list.

If the user returned then start suggesting immediately an alternative.

Blindly recommending the same thing just because there's a correlation seems stupid. Would be nice to at-least have a nicely visible button to stop recommending this item since I'm done purchasing anything similar for the next couple of years.

Amazon did literally this to me just a couple days ago.

I'd bought a vacuum cleaner on Amazon a week earlier, and, when doing an Amazon checkout of a cart with one item, it threw up a list of consumable items that I'd bought in the past, to possibly add to the order... including another one of those $120 upright vacuum cleaners.

(Maybe they have data that says this makes them more money, even though I'd guess it might hurt customer confidence in the site.)

Amazon: Sellers! Do you want to advertise to customers interested in vacuums ? Seller: YES! Take my money!

(Amazon proceeds to uprank vacuums to customers with prior vacuum sales)

I have worked on the same recommendation systems. It's also the most often oncall issue. The problem is mostly due to lag in event processing (especially orders).