The Double Overhand Knot is an extension of the regular Overhand Knot, made with one additional pass. The result is slightly larger and more difficult to untie. It is sometimes used as a stopper knot. It is more often used to tie loose ends off around the main strand of rope.
To Tie a Double Over Hand Knot:
Pass the end of the rope over itself
Around the main part of the rope once more
And then under itself and through the first loop formed