Rule: open the conversation you want to get rid of, then pick the one you want to keep.
Open the duplicate, the conversation that should disappear.
In its ⋯ menu, choose Merge with similar, then search for and click the conversation you want to keep.
Check the preview. The conversation you started from is on the left, and the arrow points to the one you picked, which is marked with a target icon. That one is the one that stays.
Click Merge.
Afterwards, the kept conversation gets the duplicate's contacts, tags, upvotes and linked tasks, plus an internal note that names the merged conversation and usually summarizes it. The duplicate's messages stay in the duplicate, which is archived and linked to the kept one. It isn't closed and keeps its status, so if it needs to read Closed in your reports, close it before you merge.
A merge can't be undone. The duplicate's contacts are added to the kept conversation and can see all of its messages, so don't merge conversations from different customers who shouldn't see each other's messages.
Example: to keep #937 and get rid of #940, open #940, choose Merge with similar, pick #937, and confirm when the preview shows the title of #940 on the left and the title of #937 on the right.
