I did a simple test.
Here is my raw data if last name is in a separate column.
Then add a row group grouped by "LastName", right-click on the newly-appeared "LastName" column, select Delete Column -> Delete Column Only.
Then right-click the cell property that you want to highlight duplicates, select "Font", and add an expression in "Bold", the result is as shown in the figure below.
Best regards,
Aniya