Write a method doubleUp that doubles the size of a list of integers by doubling-up each element in the list. Assume there's enough space in the array to double the size.

Suppose a list stores the values [1, 3, 2, 7]. After calling list.doubleUp(), the list should store the values [1, 1, 3, 3, 2, 2, 7, 7]

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