To select and delete duplicate files:
Put a check mark against files you want to delete, or press Command+M to mark/unmark the currently selected file or group.
To preview the selected file using QuickLook, press space bar. Pressing space bar again closes QuickLook. While QuickLook is active, use arrow keys to select other files in the list.
To expand all groups, click Expand All button or press Command+Right arrow.
To collapse all groups, click Collapse All button or press Command+Left arrow.
To select all duplicate files, leaving only the newest file in each group of duplicates, click Keep Newest button.
To select all duplicate files, leaving only the oldest file in each group of duplicates, click Keep Oldest button.
To delete the selected file only, click Delete button or press Command+Delete.
To delete all marked files, click Delete All Marked button or press Alt+Command+Delete.