So it seems there's two sides to this suggestion. Let's go over them.
On one side people are saying that this will make things easier for both players and staff members because players won't have to double check if they've posted already, and staff won't have to punish people for messing up. This would also help people who lag and accidentally do double post. Seems alright.
On the other side, we're saying that we shouldn't have to make extra features to accompany the mistakes of the playerbase, and it is their responsibility to not break the rules. I agree to this for the most part. It is their responsibility and we shouldn't have to do extra work to make things easier for them, right? Well... When it comes to something that's on this small of a scale, which requires something more or less as little as installing and configuring an add-on, why not?
What this add-on would do is auto-merge posts, but not just that. I'm not entirely sure if it is this particular add-on, but some of them that do this kind of thing have means to prevent constant bumping on threads due to abusing the merging system. That takes care of that issue. While players should indeed be careful not to break the rules, would it not be a bad idea to ignore something that would make both our jobs and the player's experience even slightly more seamless with as little work as it takes compared to the other things we do for the players? We've just implemented a feature that automatically locks threads after two weeks to help prevent necro posting. In my opinion, I'd rather use the time focused on dealing with necro posts and double posts to handle other things such as reports and appeals more efficiently. The less amounts of grunt work we have to do, the more we can excell in the other more impactful fields. Now, I'm not saying that we should go looking for ways to reduce our workload, but if it erases something as small and inconsequential as necro/double posting and makes even a little bit more time for the more important responsibilities of a staff member then we should absolutely implement this. Saying no to a double post merger for the reasons that people have stated so far is like not getting spam bot or DDoS prevention because it's our jobs to actively combat it, which is pretty ridiculous. I'm going to have to disagree with my colleagues and support this add-on.