Silk Touch duplication bug

I've tried Silk Touch Pickaxe with several blocks and come to a conclusion that it has some bug allowing you to break the block then gain BOTH the results if you break the block with Silk Touch and if you break the block normally.
For example:
1. If you break an Ender Chest with Silk Touch Diamond Pickaxe, it will drop:

• Ender Chest (obtained by the result of Silk Touch)
• 8 obsidians (obtained by bug and it is normally when you break Ender Chest by normal Diamond Pickaxe)
  2. If you break a Furnace with a Silk Touch Diamond Pickaxe (I haven't tried with other Silk Touch Pickaxe), it will drop 2 furnaces. The result could also be explained like above. Same thing happens with Beacon too.
  I haven't tried with a lot of blocks to see the common thing of those blocks which could be abused by this bug.