mylist = ['a', 'b', 'c']
multiplicity = {0: 3, 2: 2}

result_list = []
for i in range(len(mylist)):
    num_repetions = multiplicity.get(i, 1)
    for _ in range(num_repetions):
        result_list.append(mylist[i])

print(result_list)

Outputs ['a', 'a', 'a', 'b', 'c', 'c']

The code describes itself.