User see other user desktop - TeamViewer is installed

TeamViewer is known to create conflicting behaviors with RDP environments.

From your TeamViewer, please enable "enhanced multi user support (for terminal servers)" under advanced options, this creates a unique ID for each user that accesses the server.

Please check the TeamViewer knowledge base in this regard :

https://www.teamviewer.com/en/global/support/knowledge-base/teamviewer-classic/troubleshooting/black-screen-after-starting-a-connection/

and

https://www.teamviewer.com/it/global/support/knowledge-base/teamviewer-remote/remote-control/use-teamviewer-remote-on-windows-servers/