or, when you have added a person, did you scroll down and make sure their associated people were not added. EG if you add someone from a census, it would like to add the rest of the family group, but you can review each person and say yes or no. Tick boxes.
If you do want to delete pick on a person
then top left is the name of your tree. Select the little downward arrow
select tree settings
scroll down.
On the extreme lower right is a 'Delete Tree" option.