def METHOD_NAME(symbols):
    """Reduce expanded list of symbols.
    Parameters
    ----------
    symbols:
        list containing any chemical symbols as often as
        the atom appears in the structure.
    Returns
    -------
    reduced_symbols: any symbols appears only once.
    """
    reduced_symbols = []
    for ss in symbols:
        if not (ss in reduced_symbols):
            reduced_symbols.append(ss)
    return reduced_symbols