Averaging list of tuples based on key name?

Question

modifiedL = [(\'10-1984\', 2.8), (\'10-1984\', 2.8), (\'10-1984\', 2.85), (\'10-1984\', 2.82), (\'10-1984\', 2.78), (\'10-1984\', 2.75), (\'09-1984\', 2.82), (\'09-1