def _find_unique_columns(columns):
    """
    Find the unique columns in a list of columns.

    Example:
        >>> sorted(_find_unique_columns(["a", "b", "b", "c"]))
        ['a', 'c']

    This is necessary because duplicate column names are ambiguous.
    """
    counts = {}
    for column in columns:
        counts[column] = counts.get(column, 0) + 1
    return {column for column, count in counts.items() if count == 1}