When a question is closed as an exact duplicate, the announcement always includes the statement "its answers may be merged with another identical question". How long after the closure does this occur? Who gets assigned to do it and under what policies does this person operate?
Merging is an action that can be performed only by moderators, and this is irreversible (at least, not without getting hold of the developers to un-splice the questions). We merge duplicates very rarely, and when we do, only if it absolutely is a good thing to do. I'll explain why.
Theoretically, merging is useful for duplicates because:
However, in practice, it has a lot of issues that outweigh its utility:
So while there are questions that technically could be merged, it requires a lot of effort to first ensure that the match is pretty strong and that little to no mismatch exists between the two questions and answers (in terms of content, MWE, etc.). This certainly is time consuming and not worth the effort for little to no gain. Besides, the duplicate questions are cross-linked in the sidebar, so there is a trail from A to B and B to A.
At present, we merge only in cases such as when a question is cross-posted between Stack Overflow/Math and here, and their copy gets migrated (with answers) or if the OP repeated their question instead of editing/bountying it (typical for new users), etc. In other words, only when the two posts are more or less the same question by the same user and the double posting happend accidentally.