What is the accepted practice. Should I revise and update someone's answer to add more information, or add your own answer into the mix? For example, this question I just answered.
1$\begingroup$ You may want to look at this meta answer of mine, along with the dupe of that question. $\endgroup$– ManishEarthAug 13, 2012 at 4:28
Well, yes! Both are acceptable.
Usually, though, if you have an answer that completely answers the question and adds a significant™ — meaning the measure of such is a bit subjective — amount of information, as yours does with background and history, then it's fine.
Use your judgement and feedback you might receive from comments, etc. to decide. What we really don't want is 2-3 answers that all essentially say the same thing, as the OP would have to split hairs to decide which will be selected.
If your answer does turn out to be identical to someone else's, don't despair, though. Add additional information that's pertinent and polish it up!
I think that updating someone else's answer should be done if and only if your contribution can be distinguished from the original. Otherwise I'd advocate simply supplying another answer.
I think this way because it is too easy to change an existing answer so as to make it wrong. Who will get the credit for that?