Side-by-side page layout suddenly appeared

Suddenly whenever I create a new document it is in a side-by-side page layout. I can’t find a way to eliminate this extreme nuisance. Older documents open normally. I’ve tried comparing the Page Styles and can’t see a difference.

Text appears on a page that is offset to the right on the screen; I have to scroll to the side in order to work. As I fill the page the overflow is to a new page on the left, and I have to scroll over there.

If there is enough text it begins to appear on both sides.