Let's say there is a user who has found a satisfactory answer to a common question asked on Stack Overflow (or other Stack Exchange website). This answer may be a snippet of code, or an addon, or a framework, or something else.
Is it acceptable for this user to formulate an answer for one question, find every related question and then copy the exact answer to each of those questions? And, if not, what should users who finds themselves in this situation do?
Copying and pasting the same answer to multiple questions can cause different reactions depending on what answer you're pasting. The acceptableness of this behavior ranges from "highly frowned upon" to "completely unacceptable". (Note that it never reaches the "acceptable" end of that spectrum.) If you catch yourself or someone else doing this, please evaluate the answer and see if there is a more correct thing to do.
Are the questions duplicates of each other?
Duplicate answers most often indicate that the questions are asking the same thing. If the questions could be considered duplicates of each other, only post the answer to the best one, which you think would make the better candidate for the other one to point to. Flag or vote to close the other one so that it can be directed to the better question which has your answer.
If you spot a duplicate answer to duplicate questions, flag or vote to close one of the questions as a duplicate of the other. If the duplicate answers are at least a few days old, also flag the answer on the duplicate question for removal. (Leave the other one alone.) Most copy-paste answers are automatically flagged for moderator attention, so there is no point in flagging fresh duplicate answers; an older answer may have slipped under the radar and could have been edited to become a dupe or is different enough for the automatic flags to not have caught on.
Are they promotional in any way?
Duplicate answers across questions that promote external links or resources are considered astro-turfing and will likely result in the immediate deletion of all the answers involved. Most often, these answers get posted to questions which are only remotely related to the link or product they're trying to promote, and they often consist of little more than a vague explanation of what the link contains or what the tool does. Depending on the degree of astro-turfing and status of the account, a wide range of punishments could get issued, from a simple warning to account destruction.
Could the answers be more specific to each question?
If you've gotten past the other two questions, your answer may just be too vague. Answers that solve over-generalized problems are not as useful to questions — they should be tailored to address the specific needs that each question has outlined. If linking something, explain the relevant parts to the OP, or explain how the code or tool you've linked is relevant and useful to them. Don't just throw out answers thinking "this is relevant" — explain how it is relevant. It's not always so obvious.
Are the questions asking about the wrong thing?
Sometimes, superficially different questions may indeed be symptoms of the same underlying problem. For example, the questions may describe different errors observed in different situations, all of which have the same root cause. Or the questions may all be suffering from the X/Y problem, with the same underlying problem but different poorly attempted solutions.
Such questions are not obvious duplicates, and marking them as such without explaining why they're really about the same root issue may only leave the asker puzzled, yet they all have essentially the same fundamental answer.
If you've written a clear, detailed and comprehensive answer to one such question, explaining what the underlying problem is and how to solve it, it may be tempting to copy-paste the same answer with minimal changes to other questions that it applies to. Indeed, if you edit your answer enough to make it specifically fit each question, this may not be completely unacceptable.
That said, it's usually better to find (or ask!) a question about the actual underlying problem, and post your comprehensive answer there. You can then either vote to close the other questions as duplicates (with an explanatory comment, if it's not obvious why they share the same cause and solution) or just answer each of them with a short description of how that particular symptom arises from the shared root cause, with a link to the question about the cause for more information.
The Community user automatically flags exact duplicates
Any time you post answers which are exact duplicates of each other, the Community user will automatically raise a flag for moderator attention, in order to evaluate your answers based on the criteria we've outlined above. So please, before you get other users involved in your copy-pasting business, do the work yourself and you might be able to avoid bad situations in the first place.