def repeated_first_index(seq):
    """
    Return the index first repetition in the list.

    Return None if no repeated value is found.

    Examples:
        >>> repeated_first_index([1, 2, 1, 2])
        2
    """
    unique = []
    for i, x in enumerate(seq):
        if x in unique:
            return i
        unique.append(x)
    return None
