<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_user_flagger():
 """ Return an User instance used by the system when flagging a comment as trackback or pingback. """

    user_klass = get_user_model()
    try:
        user = user_klass.objects.get(pk=COMMENT_FLAG_USER_ID)
    except user_klass.DoesNotExist:
        try:
            user = user_klass.objects.get(
                **{user_klass.USERNAME_FIELD: FLAGGER_USERNAME})
        except user_klass.DoesNotExist:
            user = user_klass.objects.create_user(FLAGGER_USERNAME)
    return user