Should such answers be deleted as those that don't add anything to existing one?
No, there are no reasons for this.
Answers are sorted according to the vote count(by default) - this already has the effect of having the good answers at the top, while the subpar or wrong answers will be at the bottom, where they will garner less attention.
There might be a timing issue as well. For example, I could post a one line answer that covers the basics of the question. Another person comes along 30mins later and post an answer that encompasses mine but adds in a lot more details. It is not fair that I have my answer deleted in that case. It will be pretty troublesome to determine the exact order of posting.
If you like an answer, upvote it. If it isn't as detailed as you like, ignore it. If it is a bad answer, downvote it. The only time an answer should be deleted is if it is spam or abusive IMO.