no, it is duplicated. There are a lot of signs stacke on top of each other until the sign limit is reached. When you click delete on it, it deletes the sign but there is an identical one beneath it, so it looks like it hasn't been deleted. If you keep deleting them you'll eventually get rid of them all. You can't add signs because the sign limit has been reached.
Just tried, the limit of signs is 16, why there is a sign limit?