There are two windows open that show the same workbook. This is a useful
feature if you want to compare sheets in the same workbook.
To open a new window in the active workbook, choose Window>New Window
To arrange windows, choose Window>Arrange, select an option, click OK
To compare sheets, activate one of the windows, and select a
different sheet
To close the workbook with both windows open, choose File>Close
To close one of the windows, and leave the workbook open, click the
X at the top right of the worksheet window, or press Ctrl+F4.
(Don't click the X at the top right of the Excel Window)