You can drop multiples of the unique in the same game. But only because Patch 1.09 failed to address this issue; not because Blizzard changed anything. Their unchanged official position on this is that you cannot.
Item generation in Diablo 2 is never easy…
Let me try and see if I can explain what is the reason behind this “myth” is, and what is the current state of duplicate drops in Diablo 2 – which was certainly carried out to Resurrected since both share the same codebase.
There are three types of duplicate drops (not to be confused with dupes) in Diablo 2. And all are important to this discussion:
User duplicates happen when you are carrying (inventory or stash) an unique gained from another gaming session and you drop that unique from some monster or container. Blizzard never claimed this was not allowed. You can always drop a unique again, as long as it is not in the same game. But this particular type is important because of a fix in Patch 1.08 (see below).
Game duplicates happen when a unique drops more than once in the same game session. I you leave and join a new game you can drop multiples of the same unique, as long as each one drops on different games. This is the type of check that Blizzard claimed. A claim they haven’t changed.
Monster duplicates. Rarissimo, happen when a unique drops more than once from the same monster kill (usually a boss). This should be governed by the same rules as game duplicates. But as we will see below, multiple user reports of duplicate drops seem to indicate it suffers from the same hypothetical bug.
So, what has happened over the years?
Like was said before in this thread, Blizzard claimed that unique drops could only happen once in a game. But what is important to note is that, officially, Blizzard never changed that claim since.
Early on there was an unreported(?) bug in which players couldn’t drop either any unique items that they had in their inventory or stash. LoD Patch 1.08 reportedly resolved this bug and players were able to drop those uniques.
Then along come LoD patch 1.09 with this confusing release note “Fixed a bug that allowed the same Unique to be spawned more than once in a game.”
We can only speculate, but knowing that the previous patch addressed a similar issue, it seems for a brief period, between patches 1.08 and 1.09, players could get multiples of the same unique in the same game. And patch 1.09 fixed this.
But is any of this actually true in the game code?
Apparently not. Over the years there were reports of players dropping the same unique twice in the same game. These reports are many and most are just hearsay. I would probably have to struggle a bit to find actually confirmed ones. But it is fair to say we can trust the community on this and that Blizzard in fact has a bug that always, or at least since Patch 1.08, allowed players to get these drops.
Diablo 2 has very punishing drop tables. So for us users to test for duplicate drops is actually very difficult (and offline testing isn’t an option here). I can’t recall if there ever was any confirmed drop reported back to the community. For Blizzard this would be more easy to test. But I think they must have realized at some point, probably after two release notes, it simply wasn’t worth the effort.