Loss of database info

Ok, another question. I've noticed that, from time to time, a folder inside a catalog will get "lost," and the data for it disappears. The "Recent Imports" list will run out of space, and not-so-recent dates will go away. Because of that, I add folders to the folder list, and put in the folders that were made by the impost. Sometimes, these added folders suddenly show up as empty when I try to work on images in them. I tried re-importing the images to "Current location," but then the folders show double entries for each image. How can I restore the database information without re-importing the images and causing the double-entry issue?