We need a sharp definition of a dupe
I always had it quite sharp why and when I marked as a dupe, and always kept an eye on how others voted to close as a dupe. Although accidents unavoidably happen on all close reasons, I must say the inconsequense, on how voting on possible dupes is practiced, is an issue in my opinion. Not only are there many different concepts practiced on what is a dupe, even the discussion on it easily ends up in a total babel.
What I suggest
I therefore would suggest the following straightforward rules, based on "rule number one" that a good and valid question could always get an answer, and subsequently: keep all relevant information in onze place.
If two questions are the same, mention the same symptoms and it looks like possible answers should "fight" the same issue:
- Mark as a dupe.
- If OP mentions the issue is the same, but posted solutions on the dupe are not what he or she is looking for
->post additional answers on the original.
- If OP makes clear his problem is caused by another issue (or we think it is)
->retract vote and answer.
If two questions are caused by the same issue, but mention quite different symptoms:
- No dupe; the question/answer would be difficult to find. Useful to link though in the answer.
If two questions are the same, but likely caused by different issues:
->(Obviously) no dupe
If two questions are different, but one or more answers on an existing question also answer the new question:
->Not a dupe, unless the older question's title or body can be edited to include the newer question, without harming the older question.
In the rules above, the main idea is that the question defines the dupe, not the answer. The review procedure seems to support that; in review, we don't see any of the answers. The message OP gets if his or her post is marked as dupe seems to clash with that:
if the answers on the possible duplicate do not answer your question then you should edit your question to explain why.
...But it doesn't. The message mentions OP should mention why existing answer do not work for him or her, but the information is useful for new answers on the original question.
I believe in the above rules, it is guaranteed every valid question can get an answer, even if the poster is a 1- pointer, and we keep relevant information in one place as much as possible.