CAPTURE ONE FILING OF IMAGES

Hi I am relatively new to Capture 1 and I am trying to understand the filing convention used by C1 on a Mac (OSX 10.5.6). All the original images are loaded into sub-folders by date and subject in the 'pictures' folder. I am now finding some images under a Capture One Library sub-folder. Only a certain % of images are duplicated. I am trying to understand why and at what point C1 does this. Thanks in advance for your help.