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You can merge the duplicated cards using the Connect \ Remove Connection feature.
You can read about this feature here:
These are the steps you need to take, if a certain individual was entered twice into the tree:
1. Go to the card with the least amount of information, and remove all its connections to the tree, until it is left alone.
2. Delete this card, by clicking on the ‘ ’ icon, and choosing to delete person.
3. Go to the remaining card of this individual, and use the Connect feature to connect him to all the individuals that were disconnected from the deleted card.