In the Library module, if you right-click on one of the duplicates and select "goto location in Explorer/Finder", is the file duplicated on the disk?
If the duplicates are just in the catalog, is there any chance that you had a bunch of images highlighted and did a "create virtual copy" ???
On my iMac, if I right click on one of the duplicates, I do not see the option to select "go to location in Explorer/Finder". I should mention that the duplicates all seem to have the "!" mark on them.
I did select "Show in Finder" and a pop up window revealed “DSCF1020.jpg” could not be used because the original file could not be found. Would you like to locate it?". When I click on the "Locate" button it takes me to the Desktop and selects a folder, but I don't see this file anywhere.
I think I may have moved some of the original images before this started...but I don't understand why the duplicates were created.
I'm very new to Lr, so forgive my stupidity. I don't have a huge number of pics in the catalog yet because I am trying to learn how to manage workflow and keep this organized before I do any volume of work. So, this is really a learning experience.
One possibility is that you moved the photos using Finder, and you also re-imported the photos, which would produce the symptoms you describe of duplicate photos with exclamation points.
If you are new and haven't done a lot of work in Lightroom yet, you might be better off just starting over (use File->New Catalog). This will eliminate the duplicate problem, but it will also cause you to lose whatever work you have done already.
In the future, do not manage (move, rename, delete) your photos using Finder. Do all photo management of your photos from inside Lightroom. Better yet, develop a workflow that doesn't require moving or renaming photos. If you want the photo to be on a certain disk, in a certain folder, put it there straight out of the camera. Don't put it in Folder 1 and then later move it to Folder 2, just put it straight into Folder 2 right out of the camera.
You are strongly encouraged to view some tutorial videos or read some introductory material, rather than trying to figure out everything by yourself.