def remove_duplicate_words(s):
    """
    Removes all duplicate words from a string, leaving only single (first) words entries.
    :param s: a string of spaced words.
    :return: string with each word only once.
    """
    words = []
    for x in s.split():
        if x not in words:
            words.append(x)
    return " ".join(words)