you can't get a double wooden slab in your inventory

you can't get in 1.8,1.8.1 and 1.8.2-pre 1 a double wooden slab or a double stone slab with the give command but in 1.7.10 and lower its well possible

commands: /give @p minecraft:double_wooden_slab
or: /give @p minecraft:double_stone_slab