Start by clicking on the Graduation Cap (Students) in the far right sidebar.

Then click on the big action key "Manage Students" dropdown in the far right corner

Select Merge

In the search bar, put in the last name of the duplicate student

Click on the green plus for the student you wish to keep and it will move it to the top field

Click on the yellow plus for the student to merge

Click on Merge Persons

You will get an email when the merge is completed.