Prevent duplicates from itertool permutation

There is no direct way to do that in itertools. The documentation for permutations() states:

Elements are treated as unique based on their position, not on their value.

This means that though the two As look equal to you, itertools treats them as if they are not equal, since they have different positions in the original string.

The number of the results you want is called the multinomial coefficient for 4 values, 2 equal and 2 others equal. You could get what you want by coding your own equivalent function to permutations but that would take a while to code and debug. (Perhaps call it multinomial though that word refers to a number, not the actual lists.) An easier way, perhaps slower in execution and memory usage but much faster in programming, is to use permutations and Python's set to remove the duplicates. You could do this:

from itertools import permutations

perm = permutations('AABB', 4)
for i in set(perm):
    print(i)

This may result in a different order to the printout. If you want to restore the original order, use sorted(set(perm)), since permutations returns in lexicographical order (if your original string was in sorted order).