Place two slabs, use pick block on it, and then place the resulting block. It will be double smooth stone slabs, no matter the material of the original slabs.
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I can confirm this, the placed block is always the pure ID 43, without the value.
@Idea Guy, you can still get that block on creative, with "/give USERNAME 43 1 6", but when you place it you'll get the regular stone double-slab, because of the bug reported here.
I can confirm that this bug still exists as of 1.4.4.
Resolving as duplicate of MC-1959
@Idea Guy: Give yourself block 44 with a damage value of 6. This will give you the single-slab version. Just place it twice. 🙂
It's no longer possible to place the alternate double slab block (the block that uses the top/bottom texture of the stone slab on all six sides) as a result of this bug.... =(