El valor es una lista, usa append.
Código Python:
Ver originaldict = {}
lista = [5, 5, 5, 3, 6, 1, 5, 7, 1, 7, 5, 5, 3, 7, 6, 7, 2, 6, 1, 6]
for x in lista:
if x in dict:
dict[x].append(x)
else: dict[x] = [x]
>> print dict
{1: [1, 1, 1], 2: [2], 3: [3, 3], 5: [5, 5, 5, 5, 5, 5], 6: [6, 6, 6, 6], 7: [7, 7, 7, 7]}
Saludos.