Scratch Disk full

I have PSE 15.  I have never heard of scratch disks until this version.   Something is filling everything up constantly.  I can't do anything with a layout when this happens.  I've read the scratch disk tutorials but it is over my head.  I deal with 100's of images a day and make 25+ scrapbook layouts.  What is going on? I know I have duplicates of many things but how do I find them to delete?