If one of the individuals in your tree has been entered in two places by mistake, you can merge the duplicated cards using the Attach / Detach feature in Family Tree Builder.
You can read about this feature here: How can I attach / detach existing people in Family Tree Builder?
These are the steps you need to take, if a certain individual was entered twice into the tree:
1. Sort the list of individuals in your project by first name, to help you find the duplicate:
2. Go to the card with the least amount of information and carefully copy any notes, photos, citations, etc. Then delete the card by right clicking on it, and choosing Person > Delete person.
3. Go to the remaining card of this individual, and use the Attach feature to connect him to all the individuals that were disconnected from the deleted card.