A Block No Longer Giveable and Automatic Seamless Stone Slab

Upon using the command /give @p minecraft:double_stone_slab you get an error that states that minecraft:double_stone_slab is not a valid block. But, you can /setblock the double_stone_slab.
I also am curious as to the fact that when a stone stab is placed on another you get the seamless variant rather than the non-seamless variant.