The auto-check button offers a few options to quickly select the items to delete:
Tidy Up checks randomly all but one item of each duplicate group. Use this option if you do not care where the items are located. This way, just one item for each duplicate group will be left, leaving no duplicates.
Tidy Up checks all the oldest items in each duplicate group. If more than one item has the same date and is the oldest, Tidy Up will select all of them.
Tidy Up checks all the newest items in each duplicate group. If more than one item has the same date and is the newest, Tidy Up will select all of them.
It gives you the option to choose a location; it will select all the duplicates that reside in the chosen location. If all the files belonging to the same group reside in the chosen location, Tidy Up will not check one of those; this way you'll be sure that at least one item for each duplicate group will be kept.
All the checked items will be unchecked.