At one point I wrote a fairly detailed explanation of why equivalence relations on a set and partitions of that set are essentially the same thing. The issue came up in a new question again. The new question is not a duplicate of the old one, so closure as a duplicate is not appropriate. (The original question was specifically about the left cosets of a group, and the new question is more general, just complete bafflement about equivalence relations: What exacly are equivalence classes.)
It was tempting to copy most of my previous answer, which I was happy with, into a new answer to the new question. In the past I have not usually done this, and instead have left a comment pointing to the old answer. That is what I did this time. My feeling is that such cases are good opportunities to let other people write their own answers. On the other hand, I think my old answer is pretty good, and might be helpful if reposted. Although the idea of posting the same answer twice makes me uncomfortable, I am not sure why, and I have not been able to identify any specific moral or ethical problem with it.
I am interested to hear what other people in the community think about this.