If you use Google Chat on a Google Workspace account and you need classic Hangouts to chat with people outside your organization, keep both until you can switch to Google Chat. However, if you use both the Chat and classic Hangouts mobile apps on your device, you might get duplicate notifications.
Remove the classic Hangouts app
If you use Google Chat, you can remove the classic Hangouts app. Your existing and new conversations from classic Hangouts both appear in Google Chat. When you remove classic Hangouts, duplicate notifications should stop.
Use a different tool for text messages & video calls
If you used classic Hangouts for text messages, use another service, such as Android Messages. Learn how to switch from classic Hangouts to a new text message app.
If you used classic Hangouts for your video calls, switch to Google Meet. Learn how to use Google Chat to start a video meeting.