for循环，元素i之前的切片判断是否包含元素i，若包含则return结果 
class Solution:
    def duplicate(self , numbers: List[int]) -> int:
        if len(numbers) != 0:
            for i in range(len(numbers)):
                if numbers[i] in numbers[:i]:
                    return numbers[i]
        else:
            return -1