We need to define guidelines to merge questions. Who did merging on other sites before?
Which best practices we can apply here?
Moderators should be reluctant to merge without a request; however, if the questions are clearly identical (same wording / same user) or the moderator happens to be an expert on the subject matter and can confidently argue that the questions are the same, then they should be expected to act unilaterally.
Removing duplicate answers is important in terms of reducing noise, but for questions that are not yet marked CW it is doubly important as large numbers of redundant answers will risk forcing auto-CW on questions that otherwise do not need it.
Be a surgeon, not a butcher - it may be tempting to just wipe all comments on older Q&A, but resist the urge to do so unless they're clearly irrelevant ("possible duplicate", etc.); there's no revision history for comments, and zealous cleansing can lead to confusion.
It seems to me that if you're going to close a question as being a duplicate, you should be willing to (in principle at least) merge its answers with the question it is a duplicate of. If it is sufficiently different that the answers are too different, then maybe its not a duplicate?
I’m not necessarily advocating deletion, either; we want some of these merge stub questions hanging around so people can find two “identical” questions that were asked in two totally different ways. The exact, perfect duplicate question, in my experience, is much more rare than people think.