【Slack】Botについて、二回eventが送られてしまう

#やりたいこと
以下の画像のように、発言をオウム返しにするbotが二回発言してしまうのを防ぎたい。

#コード

# -*- coding: utf-8 -*-
import os
import json
import logging
import urllib.request


# ログ設定
logger = logging.getLogger()
logger.setLevel(logging.INFO)

def handle_slack_event(slack_event: dict, context) -> str:

    # 受け取ったイベント情報をCloud Watchログに出力
    logging.info(json.dumps(slack_event))

    # Event APIの認証
    if "challenge" in slack_event:
        return slack_event.get("challenge")
        
    if slack_event['event']['type'] != "message":
        return "OK"

    if slack_event['event'].get("subtype") == "channel_topic":
        return "OK"
        
    if slack_event['event'].get("subtype") == "channel_join":
        return "OK"
    
    if slack_event['event'].get("subtype") == "bot_message":
        return "OK"
     
    ary = catch_user_info(slack_event['event']['user'])
    
    # Slackにメッセージを投稿する
    post_message_to_slack_channel(slack_event['event']['text'], "important_timeline", slack_event['event']['channel'], ary[0], ary[1])
    
    return "OK"

def post_message_to_slack_channel(message: str, channel: str, channelId: str, user: str, image: str):
   
    # Slackのchat.postMessage APIを利用して投稿する
    # ヘッダーにはコンテンツタイプとボット認証トークンを付与する
    url = "https://slack.com/api/chat.postMessage"
    headers = {
        "Content-Type": "application/json; charset=UTF-8",
        "Authorization": "Bearer {0}".format(os.environ["SLACK_BOT_USER_ACCESS_TOKEN"])
    }
    data = {
        "token": os.environ["SLACK_APP_AUTH_TOKEN"],
        "channel": channel,
        "text": message,
        "username": user,
        "icon_url": image,
        "attachments": [
            {
                "fallback": "Plain-text summary of the attachment.",
                "color": "#2eb886",
                "footer": "Bot ｜ リプライはリンクをクリック→ " + "<#" + channelId + "|" + channel + ">",
                "footer_icon": "https://www.pakutaso.com/shared/img/thumb/HIRAorenobasyoosu_TP_V.jpg"
            }
        ]
    }
    req = urllib.request.Request(url, data=json.dumps(data).encode("utf-8"), method="POST", headers=headers)
    urllib.request.urlopen(req)
    
    return "OK"

def catch_user_info(user: str):
    
    url = "https://slack.com/api/users.info?token=" + os.environ["SLACK_BOT_USER_ACCESS_TOKEN"] + "&user=" + user + "&pretty=1"
    req = urllib.request.Request(url)
    res = urllib.request.urlopen(req)
    body = res.read()
    decode = json.loads(body)
    name = decode["user"]["real_name"]
    image = decode["user"]["profile"]["image_512"]
    
    return name, image