Duplicate answers are not always a problem.
As others have pointed out, duplicate answers can indicate duplicate questions. Duplicate answers can be an indication of spam or self promotion. In those cases, duplicates are not appropriate.
However, there are some cases in which duplicate answers are appropriate.
"Take a step back" answers
Take the case of several questions such as:
How do I write a regular expression to parse X out of HTML?
The questions are not duplicate because they are not asking about the same X. Rather they all deserve the same answer:
Regular expressions are not the correct tool for the job. Here is a list of cases in which they fail. Here are the correct tools you should be using to approach the problem.
If there is a well written, helpful version of this answer with links and references, it would be appropriate to duplicate it in each of the above questions.
"Don't treat the symptoms" answers
Another case would be a set of questions such as:
- Can't install X because of error
- Error when typing X on the command line
- Error in log file for certain requests
Where the error message is similar in all cases. These questions are not duplicate. They all have different context and the errors are causing different symptoms. However the same appropriate answer might apply:
The root of your problem is X. Here is how you fix it.
Again the same answer may serve multiple questions. If the answer is high quality, the duplication of the answer is not a problem in and of itself.
In one of the podcasts, Joel Spolsky, founder of StackExchange alludes to this. He says something to the effect of "Just because two questions need the same answer, it doesn't mean they are duplicate."