Help says:
Selection by Ctrl/Shift does not work. Only the clicking selects or deselects an item in the list of folders. Continuous range of items can not be selected by Shift.Dialog box: Select duplicate files
Choose folders in which files can be selected
Use Ctrl+Click or Shift+Click to select multiple files, or the Spacebar.
So it could be repaired either in the Help according to the program behaviour or in the program to work according to the Help.