My sd card makes copies of all of my photos?

Why does my SD card make copies of All of my photos? Is there any way I can stop this? I have a Nikon D3000 that I use it with and when the SD card is in the camera, its fine but when I put it into the computer it has doubles of everything. Its really annoying having to delete every other photo when I upload 50+ photos.

It's possible that you have the camera set to record every picture in two separate formats, namely JPEG and RAW. The camera thinks of it as just one picture, but the computer will see the separate files.