If you need classic Hangouts to chat with people outside your organisation, you should keep it until you can switch over to Google Chat. However, if you use both the Chat and classic Hangouts mobile apps on your device, you might get duplicate notifications.
Apply the troubleshooting approach that works for you.
Remove the classic Hangouts app
If you use both services exclusively for conversations with people inside your organisation and don't also use a non-work account with classic Hangouts, stick to Chat. If you remove classic Hangouts, duplicate notifications should stop.
Turn off classic Hangouts notifications
If you don’t talk to people outside your organisation on your work account but you do use classic Hangouts for personal use outside work, you can keep the classic Hangouts service to use with a non-Google Workspace account.
See the steps to Turn off notifications.
Sign out of classic Hangouts
Another option for stopping extra notifications is to sign out of classic Hangouts. Learn how to Sign in or out of classic Hangouts.
Use a different tool for text messaging
If you use classic Hangouts at work for text messages, use another service, such as Android Messages. See Switch from classic Hangouts to a new text messaging app.
If you use classic Hangouts for video calls, switch to Google Meet. If it’s not enabled for your organisation, you can continue using classic Hangouts.