public class Solution {
    public int[] double_(int[] numbers) {

        int[] output = new int[numbers.length * 2];

        for(int i = 0, j=0; i < numbers.length; i++){
            output[j]= numbers[i];
            output[j + 1] = numbers[i];
            j += 2;
        }

    
        return output;
    }
}