Help! I am filling in for someone and have accidently processed payroll twice. Can I undo the damage

Try going to Payroll Centre, click on Related Payroll Activities -> Edit or Void Transactions. Select the payments you want to void, click the Void button. 

That's a simple mug's answer. If your payroll is automatically linked to a payments system and that has to be rectified then that's way out of my league.