The Dual Messenger on the Samsung Galaxy Note 9 allows you to use two different accounts for one app.
For example, you can use Facebook Messenger twice for different accounts. That's great, because you don't have to log in and out all the time when using two different accounts.
In addition to Facebook, other messengers such as WhatsApp are also supported.
How to set up Dual Messenger on Samsung Galaxy Note 9 is explained in this article:
Activating and using Dual Messenger on Samsung Galaxy Note 9
1. From the start screen on Samsung Galaxy Note 9, open Settings
2. In the list, select "Advanced functions" from
3. Scroll down to "Dual Messenger"
4. Activate the slider on the apps you want to use in two separate accounts.
The corresponding app will now be installed again and is now available twice on your note 9 under "Apps".
You can now log in with different accounts on the two apps. Unfortunately, not all apps are compatible yet, but Samsung is working on it that several app developers, such as Instagram, support this.