You have perhaps inadvertently clicked on Windows, New Window on the menu.
What you describe is the Results of so doing.

"PE" wrote:

When I open a particular spreadsheet (sales.xls) two copies of it open -
Sales.xls:1 and Sales.xls:2

Whatever changes I make to :1 are automatically made to :2 and when I close
one book the other one closes as well.

What is causing this and how can I prevent this happening?

Al