As you might have found, TP does not warn you when you enter a duplicate person, even if they have exactly the same details. It should.
You cannot merge. "Merge" is not in TP's vocabulary.
Having located the duplicates, you need to delete one of them.
But you also need to examine whether either of them are are attached to parents, and/or whether they are attached to children. If so, you need to ensure the record you have chosen to remain is properly attached to the parents/children. Don't forget to salvage all the information (e.g. sources) from the record you are deleting, and to include such into the record you are saving.