If so, in what conditions? Look at the book question. It's a duplicate hell. It can't be true that they were all posted in same minutes(I didn't check). There wasn't so many duplicates when I read the whole thread a while ago. What has happened? Are we supposed not to flag them?
It's not okay to post exact duplicate answers. Usually the voting will weed out the duplicates, but not always (i.e., all content equal, vote for the post that was posted first). In general, if you're not adding to the existing value in a question, then you shouldn't be posting an answer -- this means differentiating your post from the others somehow. For example, you can duplicate an idea from another answer, but add your own unique analysis that helps the question asker.
Exact duplicate answers are difficult to curtail unless a mod does housecleaning, or users are diligent with searching for existing content (which is a difficult task in a question with a lot of answers) before posting.
Recently, a new search option was added,
inquestion, where you can search within a question (see the full syntax on the advanced search reference page). This feature is certainly not as old as the book question, nor do all users know about it, but it was added to try to cut down on duplicate answers in questions such as that one. Spread the word!
Probably enough people flagged the top book question from this thread for merging until some moderator actually did merge it.
Since, obviously, most top voted answers on the old question were the same books that are the top voted answers on every books question, the result is a mess with lots of duplicate answers.