My Nikon D5000 saves 2 copies of the same picture?

How can i stop this?
At first when i took a monochrome picture, it would save it in colour, and now it saves 2 copies of the same picture in colour.

Are you sure they're the same picture? Or are they one JPEG and another RAW?

Check your camera manual or

http://support.nikonusa.com/app/answers/detail/a_id/16431/~/user's-manual---d5000---guide-to-digital-photography

You probably have the image file format setting set to ''JPEG+RAW'', so it saves same photo in both JPEG and RAW formats. Check your manual and change to JPEG.