If you have added someone twice by mistake, you should be able to click 'Change or delete' in the left hand panel and remove that person.
If you are seeing boxes marked 'Duplicate' they indicate that there are two different relationships between the person selected and the duplicated person. For this reason, you can't delete those boxes, but clicking on someone else may mean that they don't show. There is still only one set of information stored for each individual.
