When creating an Advanced List, users may have multiple rows appear with the same information. This is due to some of the objects used in the list having more than one entry that would display on a line if the field was selected to show.
In order to make the list collapse duplicate rows, mark the box Enable Grouping Options on the Display fields Tab.
After enabling this, leave the Grouping column set to Group By for all display fields. This will group any duplicate rows together, removing duplicates.