When someone improves your answer to the point of providing an additional 2/3 to it, should they, or should they not just offer up their own answer? I researched (what turned out to be an incomplete tally) my answer to the question and posted it only to later find a much more complete version had been edited in. Now I'm sheepishly getting upvotes when 2/3 of the credit belongs to someone else.
One of the recommendations is to work towards "a definitive answer", by editing an almost complete answer into a great answer - that's also why you can mark your answer as "Community Wiki" - where you're effectively saying "help me answer this".
If another user has added a substantial amount to your answer they probably felt that having one complete answer was better than two incomplete ones - after all you can only accept one answer.
If you're feeling guilty about earning the rep on the back of someone else's work, then certainly mark your answer as C/W, that way you'll stop earning rep for it.