It must have something to do with either the file name, the metadata of the file (like the date and time on you camera is not set correctly or reverts to some Start Date which is the same for the other images) or a combination of both (or some other factor that is making newly shot images look the same as older images to LR).
It could also be a permissions problem. Your username doesn't have the proper Rights to Write files to the Destination location you have selected for Copying image to.
So check your camera/cameras to see if the Date, Time and Time Zone are all set correctly and the same and that you are recording images in consecutive file naming/numbering and not Repeating file name/numbers.
Then check the Destination location for the Read Write permissions for your username (and or select a different destination location that does have proper permissions).
I suspect your camera's file naming sequence has been reset so new images are being named the same as previous images. Lightroom sees the name on your computer and the name on the card so considers them duplicates.
If you know the images are not duplicates, uncheck the "Don't import suspected duplicates" box.