The host will see multiple or cascading windows, as shown below if they join their own session while they are presenting.
This occurs because the host is sharing their screen and when they join their session as a guest, they see the view of their own screen in the guest viewer, which then becomes part of what they are sharing as the host thus creating the endless window effect. It is similar to standing between two mirrors which reflect your image back and fourth creating many images of yourself as the mirrors reflect into each other.
To stop this once it has happened, simply close the viewer window by clicking the 'X' button in the upper right-most window. (See image below.)
To prevent this from happening, hosts should never join their own session from the same computer they are hosting from.
If you need to do some testing, join the session from another computer or have a colleague, friend or family member join your session to carry out the tests.
Click the upper-most right 'X' (arrowed above) to close the viewer and end the cascading windows.