Macbook Screen splitting horizontally into two,

While working on MS Word on macbook pro, Big Sur; because of some key combination, the screen is splitting into two horizontally. When I move the cursor, it moves in both of the split windows, is very slow and it is difficult to control it or work in this situation.

When I restart the system it returns to a single screen.

Does anybody know what key combination is causing this problem.?

Is there any other method or shortcut key to exit the split window, without restarting the system?

Thanks