Continuing the discussion from Moving posts into an existing topic doesn't keep chronology:
I don’t agree with any of the options proposed to resolve how to merge posts and my comments have moved too far from the topic to reply within it.
Don’t merge topics at all. Come up with another solution.
IMO, at the risk of upsetting others because I don’t mind expressing this strongly, there is no generally acceptable method to merge two or more conversations without bastardising the merged conversations in some way. I see the issue as very clear cut. I’m also optimistic that there is a better range of possible solutions.
It’s similar to the practice of photoshopping (merging) people - the absent, the dead, the imaginary - into a family photo. You can do it but it isn’t generally a good idea and doesn’t feel right. Although, I guess early photography was built on preserving the memory of dead people - after all, they were the only ones who sit perfectly still for the full exposure. 
So maybe this is not a good example but it expresses the idea that this feels wrong. 
Where do you stop?
If you have to do it for two conversations then you potentially have to do it for many. The more posts that are merged then the more issues can be expected to emerge.
What happens when further new posts are created on the same topic? A better and more logical solution would be to beef up the moderation or to improve the automated topic checking at the point of creating a new topic. For example, inputting a new topic title and searching could be combined more than once up until the posting of the new topic. That would give people more than one opportunity to realise there is a better path to take.
There are better things to do
How do you decide which topic remains? When combining topics of similar quality and merit this becomes a rather arbitrary exercise which will reduce the apparent contribution of one or more of the users who started the topics. That’s not fair and may prove to be wrong if merging topics is made very easy.
My assumption is that the conversations should be preserved as they occurred. If preservation is no longer a key driver then there are better things to do than just merge topics.
I’ve have seen many topics that add little or nothing to the quality of the discussion in a forum. If there is moderation of conversation quality then something should be done to reduce the noise and I would hide, archive or delete those topics. If there is no moderation for quality of discussion by such methods then why is there such a need to merge topics that are useful and similar?
There are better solutions e.g. wiki
The solutions proposed don’t seem to be required for the problems documented here:
If the advice or solution needs to be in one place or is too important to repeat why not wiki it in a new topic? Even then, some of the reasons for multiple topics will still occur - e.g. didn’t search for the right terms - but at least there is a proper solution to save typing.
Is the process of resolving an issue less important than the solution? As long as the two topics are cross-linked then anyone can see what is happening. Some people will learn from the journey as much as the final solution. Merging the topics loses/obscures/denigrates that journey.
Like the original issue, there are solution posts that superficially appear to be the same or nearly so but which are significantly or radically different. To merge them and reduce the solution to one topic may lose the subtle or deep differences.
Again, if the correct solution is desired then it should be in a wiki too more clearly help anyone else with the same sort of issue. There is then one point of reference for a comprehensive description of the solution that can be linked to for every new topic on ths same issue.
The proposed solutions are worse than the problem
I do not want a batch of old comments to appear as new comments even if they don’t appear in the latest notifications. The topics still has great potential to be confusing and it will not be obvious why unless it is signposted as merged. I won’t necessarily join it at the start or read the end so some alert would need to be visible everywhere in the merged topic. Why should the discussion be converted into a sort of email-like melange when the two different topics make sense standing on their own?
I would also find it confusing to have two topics merged in an interleaved manner. I read the topic as it occurs and don’t want to read it as someone else wants it to be.
People do not stop making mistakes, don’t even realise that they have made mistakes, and commonly produce confusing behaviour. Why is there a requirement to correct this? If it is for clarity then there are better solutions. If it is for tidiness then the topics would be better fit into a hierarchy of categories using another solution than Discourse.
Feature request example
The ability to merge forum topics, especially in feature requests, the same question or idea gets posted multiple time.
I copied this from a web search because it makes a more compelling case. In a category of feature requests it makes good sense that each feature should only be documented once. But there are many reasons why it should not: The main one I want to focus on is that the superficially similar topics actual cater for different people who, for example, use different language and word pictures. If they don’t know the correct terminology or way to describe the issue or feature request then there is a good chance that someone else doesn’t either. Retaining the posts is useful but how do you retain the original flavour.
You “need” to create a new post to retain the original topic description. If you don’t retain that then why not just delete the repeating posts?
Anyway, one solution is that there can be more than one category for feature requests. A plebeian/lay category where suggestions are made then a canonical/professional category where the requests are refined and single features combined by the “priesthood”.
But even with features more clearly delimited there will generally be overlapping scope, shared descriptions and other interdependencies. Merging request topics does not remove this issue.
