def unduplicate_field_names(field_names):
    """Append a number to duplicate field names to make them unique. """
    res = []
    for k in field_names:
        if k in res:
            i = 1
            while k + '_' + str(i) in res:
                i += 1
            k += '_' + str(i)
        res.append(k)
    return res
