To select and delete duplicate files:
Put a check mark against files you want to delete, or press ⌘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, choose View→Expand All command from the menu, or press ⌘→.
To collapse all groups, choose View→Collapse All command from the menu or press ⌘←.
To select all duplicate files, leaving only the newest file in each group of duplicates, choose Keep Newest command from the Select menu button.
To automatically select all duplicate files that should be deleted, leaving the newest file in each group, or in case of images, preferring Aperture files to iPhoto files, and iPhoto files to ordinary files, click Select Duplicates button, or choose Select→Select Duplicates from the menu, or press ⇧⌘A.
To delete all selected files, click Delete Selected Files button or press ⌘⌫.