Where are you seeing these duplicate people? Are there 2 people of the same name in the list of all people? Or, are you looking at the Family View Display?
When you add these records, are you being certain to use add to someone in my tree, rather than allowing the record to create a new person?
And, if you do truly have duplicates of the same person in your tree, there is now a "merge duplicates" option, making it much simpler to eliminate any duplicates accidentally created.