This is standard practice within the diamond moderator mandate.
This happens rarely though. We do this when there's a clear and urgent need, or when a community decision was in clear error. This community has its stuff sorted out so this kind of situation doesn't present itself more than once in a blue moon.
This was one of those situations: an error was made, and I'm sorry to say that, though I'm sure it was with good intentions. In this case, closing that question as a duplicate was incorrect: although they are two questions about handling Ettin semi-beheadedness, our duplicate votes are for when two questions are the exact same question, not merely covering different sides of the same topic. I think we can agree that “do they take bleeding damage for this?” is not the same question as “do they lose control over half their body?”, right?
The stated purpose of the duplicate close votes is to collect the answers to the same questions in one place so that people don't have to go looking. But a person looking for whether an Ettin maintains control over their body isn't going to be helped by answers telling them whether the Ettin takes bleeding damage.
SevenSidedDie reopened the question on approximately that basis. I'd been keeping my eye on the question myself, and planned to do exactly the same if SevenSidedDie hadn't beaten me to it.
Now, if there was a singular “So how do we handle an Ettin losing their head?” canonical question that covered various niche scenarios like this, both of those questions would be viable duplicates of it. We don't have one of those questions yet though, and right now what that person's doing is OK. It's fine to have multiple separate questions exploring a specific confusing phenomenon — it's how we get all our polymorph questions for instance.
Or should a mod begin a reopen vote and let the community decide if it's worthy to reopen or should stay closed instead of overriding a community decision?
We actually cannot do that. Diamond moderators get a unilateral close/reopen vote power and that power doesn't come with an off switch. It's why you'll see our close votes so rarely and often at the end of the list (unless it was from before we became diamond moderators) — we only add our votes when we're sure, and whatever we vote for happens immediately.