Here's my two methods on this. The first works with a list, maintains the found order of the hashtags and takes care of duplicates. The second works with a set, then returns the set converted to a list.

def extract_hashtags(tags):
    """Return a list of all mentions in text, converted to lowercase,
    with duplicates not included
    """
    hashtags = []
    for word in tags.lower().split():
        if word.startswith('#'):
            hashtag = word[1:]
            if hashtag not in hashtags:
                hashtags.append(hashtag)
    return hashtags

def extract_hashtags(tags):
    # return list({word[1:] for word in tags.lower().split() if word.startswith('#')})
    # The above will do it all in one line
    # Here's the longer version
    hashtags = set() # sets automatically remove duplication
    for word in tags.lower().split():
        if word.startswith('#'):
            hashtags.add(word[1:])
    return list(hashtags)

Hope these help.