Binoculars (by 3 users)

Gently squeeze the two barrells together (they should be able able to do it easily). I believe it's called the interpupilar distance. If you can't get the binocs to do it, take them back to where you bought them.
I admit I bought a cheap pair on the internet on which the barrels do move, but they don't get close enough to totally eliminate the double vision thing. No wonder I got them at a decent price.