It is not possible to merge duplicate person cards. If one of the individuals in your tree has been entered in two places, you can delete one person card and keep the second person card with the help of the 'connect/disconnect' feature.
These are the steps you need to take, if a certain individual was entered twice into the tree:
1. Find the person who appears twice in the tree, and open his/her card with the least amount of information.
2. Remove connections between this card and other family members in the tree as described in the related article, until it’s left alone.
3. Delete this card, by clicking on the ‘More’ button on the left side panel, and choosing ‘Delete this person’.
4. Go to the remaining card of this individual, and connect him/her to all the individuals that were disconnected from the deleted card.
Please use the following FAQ article for the detailed instructions on how to remove the connection and connect individuals in your family tree online: How can I connect / disconnect existing people in my online family tree?
Note: a person’s card may appear twice because of cyclical relations inside the tree, but, in that case, there will be an ‘x2 icon’ next to him, meaning the data is NOT duplicated.