>>> a = ["aa","bb","cc","aa","dd","ee","aa","aa","ff","aa"]
>>> a
['aa', 'bb', 'cc', 'aa', 'dd', 'ee', 'aa', 'aa', 'ff', 'aa']
>>> a.index("aa") ## 返回单元元素的索引
0
>>> c = a.count("aa")
>>> c
5
>>> b = -1
>>> for i in range(c):
b = a.index("aa",b + 1,len(a))
print("aa",b) ## 返回重复元素的所有索引
aa 0
aa 3
aa 6
aa 7
aa 9