python 列表 輸出重複值以及對應的角標

a = [99,1,2,1,3,4]
# 集合存儲重複數據
b=set()
for i in a:
    if a.count(i)>1:
        b.update([i])
dict ={}
# 遍歷重複數據
for j in b:
    list = []
    for A in range(len(a)):
        if j == a[A]:
            list.append(A)

    dict[j] = list
print(dict)