Java Edition:Double Stone Slab (Item)
The double slabs are made by placing two slabs. They normally cannot be obtained as an item. Despite that, there have been a couple of ways to obtain this item.
Double slab items are a deleted discontinued feature, since they are removed from the game completely in 14w25a.
Obtaining
Beta 1.6 Test Build 3 Debug Chests
In Beta 1.6 Test Build 3, any newly generated world would contain up to 11 debug chests with every item in the game. The double stone slab item was obtainable in the chests 2nd and 5th from the left in the 12th and 23rd slots respectively, in stacks of 64.
Silk Touch
In Beta 1.9 Prerelease 4, Silk Touch allowed for many more blocks than it does currently, and this included double slabs. Simply stack two slabs on top of each other and mine them with a Silk Touch pickaxe. Attempting this with wooden slabs will also result in the stone variant dropping. This is because in this version wooden slabs were just a different damage value of stone slabs.