I took one picture but two photos comes out?

I'm having a Nikon D7000, using two 8GB Nikon's SD card. Every time i take a picture, two picture comes out. It doesnot show in my camera but when i put my SD card into my computer and view the photos, for every picture i take, the exact same picture all appear next to it and i have to delete it one by one. The more annoying thing is that afterter deleting all the extra picture, from a full 8GB SD card, it's now only use up less then 2 GB. It's eating my memory lol. What can i do about it?

First, read the instructions that came with your camera.

Second, read the instructions that came with your camera.

Third, repeat the first two steps.

And the chapter you should concentrate on is photo quality settings. The bit about jpg and nef.

It sounds like you're shooting in RAW + JPEG. To change that, hit the Menu button, then scroll until you're in the Shooting Menu. You should see an option under there that says Image quality. Select that, and you can choose from a variety of options. You'll want to choose either JPEG fine, JPEG normal, or JPEG basic to maximize how many photos you can put on a single memory card.

If you ever decide to do post processing using programs like Lightroom, you may want to switch back to RAW since JPEGs created by the camera lose so much of the original data.

You are shooting in RAW+JPEG mode. You can change that in your quality settings.

As far as why you are missing space on your card, it is hard to say. My guess is because you are deleting the images as opposed to formatting the card. I format the card every single time I put it in to the camera.

I use a Nikon too and it can be set up to make a nef copy and a jpg copy…
just fiddle around in the picture quality settings and you should find it

You are shooting in RAW+jpeg mode. So the camera is saving a RAW file and one jpeg. Just change the mode. Look in your manual for instructions on how to do this.