You have two options for making them the same person:
Partner with someone already in tree (via the "Add partner/ex" button)
Set Parents to someone already in the tree (via the "Change or delete" then "change parents" buttons)
You will delete one of the identical people, then add them back at that spot by picking their spouse or child and using one of the methods above.
That's not the best explanation. If it doesn't make sense, let me know.