Click "+" button and select "Keep Only ..." menu. Then select its "Unique Rows" submenu.
You can also select "Keep Only ..." from the column menu of the column(s) which you want to be unique in the result, and then select "Unique Rows" from its submenu.
Select the column to make unique in the result from "Based on These Columns" column selector.
Specify whether to keep other columns in the result with "Keep Other Columns" dropdown.