I added a person twice to the family tree. How do I delete this error.

Merge the 2 profiles.

@AHeather5 If you are the only person who has updated either one of them, you can just delete it. There is a Delete Person button near the bottom of the Tools section on the right side of the profile page. The button will be disabled if another user has updated the profile. If it is disabled for both profiles, then your only option is to merge them.