I’ve seen a number of threads on this topic, with a number of workarounds, most of which seem to no longer work-around.

I recently bought a book, and there is absolutely nothing about the marketplace that indicates to me that I own this book in order to prevent me from buying it again. Nor does the filter “Show Items I own” work in the slightest. (It shows items I own whether I select this or not.) At one point I bought a bundle that doesn’t exist anymore. I still have access to the books from that bundle, but no way to tell what that set was, other than to attempt to access each book in turn, and make notes on what I have access to and what I don’t.

Because the marketplace doesn’t prevent me from buying multiple digital copies of the same book, I am loathe to even use it.

I presume, six years or so since people started clamoring for this functionality, that D&D Beyond is simply hoping to scam people for multiple purchases by accident?

Or is it me? Am I missing some feature in the marketplace?

Every book or bundle I click on that I happen to own, has "you already own this" above the price, right above the add to cart button. 

Quote from Blunt_Jackson >>

Nor does the filter “Show Items I own” work in the slightest.

Well, it works for me just fine

The marketplace doesn’t restrict you from purchasing content you already own so you can purchase it as gifts for people.