In order to make the list collapse duplicate rows, mark the box Enable Grouping Options on the Display fields Tab.

Image of the Enable grouping options checkbox

After enabling this, leave the Grouping column set to Group By for all display fields. This will group any duplicate rows together, removing duplicates.

The "Group By" option that you see when you mark enable grouping options is removing what are considered "True Duplicates" in a list. If you have a list that is using display fields for objects that you have pulled, if any of those fields are considered a "One to Many" field - meaning that field has more than one result possible based on the information being displayed - it's going to cause that individual to show up more than once in the results of that list. Group by is going to take those results and flatten them to a single entry. As an example, here is a basic list that contains Users and Address information:

The only three fields being displayed are First Name, Last Name, and City - yet the user we see here is displaying twice, despite all the information being displayed being the same. The reason is because the "User Address" object is pulling the "City" field, and while City is the same, it doesn't mean the other fields within the User Address Object are the same as well. Because the values are actually different, we see the user twice.

This can be demonstrated if we add additional Address Data to the list: 

After adding in some additional fields such as State, Zip, and Address we see that these are actually two different values which is why the user was showing up twice despite the results being exactly the same. If we mark "Enable Grouping Options" in Display Fields - it detects these fields that are marked as "Group By" and if the values are the same, it merges the two together. Here's the same list from the first screenshot with "Enable Grouping Options" Marked: