python 列表输出重复值以及对应的角标方法

a = [99,1,2,1,3,4]# 集合存储重复数据b=set()for i in a:  if a.count(i)>1:    b.update([i])dict ={}# 遍历重复数据for j in b:  list = []  for A in range(len(a)):    if j == a[A]:      list.append(A)  dict[j] = listprint(dict)