In Python, you can achieve this using a dictionary to store the indices of each element. Here’s a simple function that does what you want:

def find_matching_indices(lst):
    index_dict = {}
    for i, item in enumerate(lst):
        if item in index_dict:
            index_dict[item].append(i)
        else:
            index_dict[item] = [i]

    result = list(index_dict.values())
    return result

# Example usage
input_list = ['a', 'b', 'c', 'a', 'a', 'b']
output = find_matching_indices(input_list)
print(output)

This will output [[0, 3, 4], [1, 5]], which is the desired result for your example.

Alternatively, if you are using NumPy, you can achieve the same result using the numpy.where function:

import numpy as np

input_array = np.array(['a', 'b', 'c', 'a', 'a', 'b'])
output = [np.where(input_array == item)[0].tolist() for item in np.unique(input_array)]
print(output)

This will also give you the same result.

Note: The NumPy approach might be more suitable if you’re working with larger arrays or need additional functionality, but for small lists, the first approach using a dictionary is straightforward and efficient.