Should we answer each of these individually at all?
If you think a question is a duplicate, vote to close it. Related questions are not closed as duplicates, though.
Could we have a mechanism to merge them into a single generic question?
Questions that could be possibly merged are the ones that are exactly duplicates. For related/similar questions, we could create a canonical question used to answer similar/related questions, where the answer is used as comprehensive answer for any related question, when creating such question is possible, or worth doing it. Merging a question asking about saying working for/working at with a question about working at/working in would not make much sense.
Should we advise posters to avoid the practice?
What we can do is pointing the OP to the duplicate question, when a duplicate question exists; in the other cases, we can point to any related question. Apart that, I don't think we can say to the askers not to ask a question because is related to another question already asked.