When looking for duplicates, remember that the computer may use files in different locations for different reasons. For example, there may be a file called "thumbs.db" in more than one folder as these are cached thumbnails for Thumbnail view in Explorer. These duplicates happen to be safe to delete (they will just be recreated when you next use Thumbnail view), but other files may not be so easy to delete.
Deleting the wrong files can cause programs to stop working and may even ruin Windows itself. Always keep a backup of your computer, make sure you can boot using a disk or CD, and always move files instead of deleting them. If something goes wrong, you can copy the file back and things will get better. Some programs keep their own files in case the main file is altered. Therefore, just copying the duplicate may still not work.
Above all, be careful, use a well known disk clean up program and if possible, try some way to increase the size of your hard drive rather than deleting stuff you may still need.
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