More details would be helpful. What device is it? Are you getting this for all notifications for all apps, or just some apps, or one app? What troubleshooting steps have you tried so far? When did it start doing this?
I'm using 11 Pro Max and I've noticed this behaviour a couple of days ago. I tried hard reset but unfortunately that did not help. I don't know if this annoyance exhibits across all apps but I can safely tell that the ones that were ok now producing two identical notifications instead of one.
I'd suggest updating to the latest iOS version (13.6 13.6.1) and try toggling notifications for those apps. If that doesn't resolve it, try resetting all settings (Settings > General > Reset > Reset All Settings).
I just experienced this issue with Teams notifications this morning. I'm chalking this up to a server-side issue for now instead of resetting my device since resetting is a pain.
Updating to 13.6.1 now and see if the issue is still there.