def find_repeat_num(arr):

    count_dict = {}

    for num in arr:

        if num in count_dict:

            return num

        count_dict[num] = 1

    return None



# 示例

arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

result = find_repeat_num(arr)

print(result)  # 输出：1