When using pickblock on a double slab of any kind other than stone, the block will be placed as a double stone slab.

Take out any slab (other than stone) place the slab on the ground, place another slab on top of the first (to create a double slab) remove the slab from your inventory, use pick-block on the double slab block you just created, place the block that was given to you, it is now a double-stone-slab block.