How do I duplicate blocks?

I have been told that there's an easy way to duplicate blocks, but not how to do it. Is there an easy way to do this?

You can do this only in creative mode.

If you are in creative mode selecting(clicking) a block with the pick block key (default middle mouse button) should give you a stack of that block.

This video shows how to duplicate blocks easily in survival mode.

Just throw the block and don't pick it up. Save and quit when its on the ground. Pick it up then use the 'X' on the Minecraft window to exit on the save and quit menu. Log back in and you'll have duplicated the block.