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 this card by clicking on the ‘Edit’ button, and then selecting ‘Person’ and ‘Delete person’.
3. Go to the remaining card of this individual, and use the ‘Attach’ feature to connect him/her to all the individuals that were disconnected from the deleted card.
You can read about this feature here: How can I attach / detach existing people in Family Tree Builder?