I have two lists that I would like to associate by index as key value pairs in a dictionary. The key list has multiple identical elements. I would like the all elements in the value list to be paired as a list of list. I am using the list.append() method, but this is not giving me the desired output. Any recommendations on the code or should I be looking at the problem in a different way?

```
list1 = ['a', 'b', 'b', 'b', 'c']
list2 = [['1', '2', '3'], ['4', '5', '6'], [ '7', '8', '9'], ['10', '11', '12'], ['13', '14', '15']]

combo = {}
for i in range(len(list1)):
    if list1[i] in combo:
        combo[list1[i]].append(list2[i])
    else:
        combo[list1[i]] = list2[i]

```

Current output:

```
{'a': ['1', '2', '3'], 'b': ['4', '5', '6', [ '7', '8', '9'], ['10', '11', '12']], 'c': ['13', '14', 15']}

```

Desired output:

```
{'a': [['1', '2', '3']], 'b': [['4', '5', '6'], [ '7', '8', '9'], ['10', '11', '12']], 'c': [['13', '14', 15']]}

```