Double the value of array on a match - Python

Question

I want to iterate through the elements in the array and double the value of the number whenever an element equals a number. Also need to find the maximum number.
Here is my code which is incorrect.

def doubnum(arr,num):
  for i in arr:
    if i==num:
    num=num*2
  return num
doubnum([1,2,4,11,12,8],2)

 Any help will be greatly appreciated.