Sometimes a duplicate question gets an answer before it gets closed as a duplicate. In some cases this answer is either better than the answer(s) in the "main" question, or at least adds something valuable to them. Such a situation can be very annoying, because it is not clear what future duplicate questions should be made duplicate of, information gets scattered, etc.
Technically, such a duplicate question can be merged into the "main" one; this copies all the answers into the "main" one, as if they were there all along. Only moderators can perform such merging. It is not clear to me what guidelines moderators use to decide on merging, and I would be grateful for explanations.
Here is one example:
- Main question: Using principal component analysis (PCA) for feature selection
- A recent duplicate with a decent answer: PCA Feature Selection
- An old duplicate with a decent answer: Using principal component analysis (PCA) for feature selection in regression
I flagged the recent duplicate for moderators attention suggesting a merge; my flag has been marked as "helpful", but nothing else happened. What does it mean?
I did not flag the old duplicate, as I am less sure that it should be merged. E.g. the answer there is accepted; I guess in case of a merge, the answerer would loose the reputation associated with having an answer accepted, which is a pity. Still, reputation issues aside, I think it would be more convenient if this all was merged.