What to do?
We don't discourage duplicating answers, because they're never actually exact duplicates. Differences in phrasing, organisation, or reasoning can make two answers that seem like duplicates quite different, and the votes will show the difference. The only kind of duplication we discourage is when it's so short that it can't possible add anything useful over the existing answers. (Those usually don't get deleted, but they do get downvoted into obscurity.)
So in this case, submitting your own answer that's complete (2) is one of the preferred courses of action, as it has no actual con and will probably be a better answer. You can leave a comment instead (3) if you like, but you're right that the author may ignore it; if they do you can leave your own answer, if you remember to return to that question.
Editing the answer (4) is sometimes appropriate, but that's rare and you'll generally know it when you see it. Most often a new answer is better so the reputation goes to the person who actually did the work, not just whoever posted an incomplete answer first.
The least helpful is (1), because then the information is split across the answers. At which point, a third person would be justified in coming along and writing a complete answer to solve that division.