If you come across a question that can be answered with one of your old answers, what is the policy for re-using your old responses? (assuming the the question is different enough or too old to be closed as a duplicate).
While I see nothing wrong with re-using old answers on new questions, the likelihood that the answer is a perfect fit for both questions is close to zero, unless you're deliberately trying to astroturf a product, or the questions are exact duplicates.
Always provide a custom written answer for each question (even if it contains substantially the same information as an old answer you have written), and you'll never have a problem.
Copy in part occasionally? A-Okay.
Copy in part frequently? Consider a canonical question.
Copy an entire answer verbatim? Comment and close as duplicate.
Copy in Part Occasionally
Alice asks a question, 'How do I get from A to D?' It is a good question that follows all the rules, so IQAndreas creates a thoughtful answer explaining how to take the ideal path from A to C to D.
Bob asks a question, 'How do I get from B to D?' Another good question, IQAndreas explains how to take the ideal path from B to C, and then copy-pastes the portion from C-D from the other answer since the process is identical.
Copy in Part Frequently
If the process of getting from C to D is popping up very frequently, but doesn't have a question of its own, you may want to create a canonical question explaining how to get from C to D. Hopefully this will teach people how to solve half their problem, and limit future questions to getting to C. If a question does ask how to get to D, you can explain how to get to C, and then link to the process for C to D.
Copy Entire Answer Verbatim
If, on the other hand, Chris asks, 'How do I get from A' to D?', and the answer for how to get from A to D applies with no modifications, then it should probably be closed as a duplicate rather than copy-pasting the answer. In the meantime, a comment saying, 'This explanation of how to get from A to D should solve your problem' (possibly with an added comment explaining why A and A' don't change the solution) will help the asker get a quicker response.
If it was someone else's answer, I'd provide attribution, but providing attribution to your own answer may look like "I answered this question before:
link-to-answer. But here is that exact same information again: ...".
I see this as a good argument for just copying the relevant information from your old answer, and editing it to suit the new question where needed.
The only reason I see to include a link to your own original answer is if there are more answers on the linked question that they can benefit from (in which case, the link would fit better in a comment than an answer).
As I mentioned in the comments, I've re-used content from past answers in other answers. For instance, I cannibalized one of my old answers in writing this answer. The questions were different enough that it didn't seem appropriate to close as a duplicate, but the background explanation was the same. The difference was in how it was applied to the problem at hand. I did make a point to note that I was doing it though, and linked to the other answer.
The problem with some of the situations where this arises, though, is that there's a very common thread to certain questions, and most of the explanation needed to show a question asker what's going on is the same. The details of how something applies in the particular case is different though. Recently I've seen this in
- application: not a procedure (and some similar questions)
In some cases of these kind of things, I flag as a duplicate, but add a separate comment (because comments indicating duplicates often get deleted, grumble) indicating how that particular answer relates to the present question. What I'd really like to do in these cases, though, is to write a new question with a canonical answer and then close the rest as duplicates of it. This isn't really an option, though, when some might already have answers or be closed as duplicates of each other.
The other problem in writing a canonical question/answer for a particular error or task is that by the time you generalize the problem, it's no longer a specific programming problem, and so it's not likely to be on-topic for Stack Overflow anymore. That's why I favor the "close as duplicate, but include a comment about how to apply the answer to the present question" approach.