python List Output Duplicate Values and Corresponding Corner Methods

As follows:

a = [99,1,2,1,3,4]
#  Collections store duplicate data 
b=set()
for i in a:
  if a.count(i)>1:
    b.update([i])
dict ={}
#  Traversing duplicate data 
for j in b:
  list = []
  for A in range(len(a)):
    if j == a[A]:
      list.append(A)

  dict[j] = list
print(dict)