The source you selecting for the import in Lightroom, you need check does that have any old images saved which were already imported in Lightroom.
"Do not import Duplicates" this option helps you to detect the duplicate images from the source you have selected to import images.
Hope that helps.
Thanks for the reply. I really believe that I have exceeded the abilities of Lightroom to determine if it already has the image or not. I have a complex workflow with several photographers and several catalogs, as I try to merge the data from a shoot, 25000 images, lightroom is unable to determine which images it already has. I am pulling images from memory cards, and hard disks used by the other lightroom catalogs. They may be some images on the cards or disks that I already have in my catalog. I am hoping that Lightroom can determine if I already have them, but in many cases, Lightroom thinks they are new, and after I do the import, it shows the hundreds of images as duplicates. If it knows that once the import is done they are duplicates, why did it now know this prior to importing. I then have to go thru the catalog and remove the duplicates. For a few images, no problems. When dealing with thousands of images, time and disk space become a problem. I think the Adobe engineers need to revisit the code for image duplication detection on import to over come this problem.