If you are referring to your answer that would be edited to include what reported in the other answer, then I don't see any reason for including what already written in the other answer. If you agree with what said in the other answer, you could add a note in your answer, saying that instead of repeating the other answer, although that could be a comment for the other answer.
It would make sense to add to your post what already said in the other one, if your answer is more restrictive of what already reported (e.g., "it's A when B; it is C when D" where A is what reported in the other answer) or for example, when the question is about the meaning of a word (or an abbreviation) that has four meanings, and the other answer reports just two of them. It also make sense if your answer is referring to what reported from a dictionary, when the other answer didn't have any reference, or when your answer is taking as reference a dictionary that was not referred from the other post.
In the cases I described, you should be the one who answered for second.
I am not saying that what being the first to answer, and editing your answer basing on what reported by other posts is wrong, but I imagine what would happen if a third user would include what already said from the first answers, where the first answer has been edited to include the second answer. Users would read three different answer but that basically are the copy of each other.
If by "proposed answer" you mean the user has written as comment what should have been an answer (I cannot see deleted comment, and I am trying to guess what could be happened), then I would give the user time to write an answer out of that comment; if then the user doesn't write an answer, I would add what reported in the comment in my answer, giving credit to the user who commented.