How to double anything:
First, go to your house and take out anything you want to double. Take out cobblestone or dirt for next step (you'll need 69 of eigther)
Next, go right outside your house and build with your cobblestone or dirt, upwards!
When you can't go any higher, jump down (you will die, you have to for this to work)
Once you die click "Respawn" and find the things you dropped when you died.
Once you pick them up, go very FAR away drom where you died.
Go back to your homescreen and delete minecraft off your history.
Go back in and find where you jumpped.
You now have double!!!
Sometimes this will not work, if you have doubled too many times.
PS. I checked this on my iPad... Im not sure if it works on a PC.