The bug

If you stack two cut copper slabs on top of each other and then mine the resulting block, only one cut copper slab item is dropped. For other types of slabs (like cobblestone), two slabs are dropped. Every variant of cut copper slabs is affected. The level of weathering and whether or not the block is waxed does not make a difference.

To reproduce

Please see this video: Double copper slabs only drop a single slab when mined.mp4