check for multiple instances of worksheet or window

[Laavista]

I'm using Excel 2007. I have a situation where users often open a 'new window', then 'arrange all' to compare data in different worksheets in the same workbook. They sometimes save the workbook with the 'new window' still open.

An error occurs in the VBA code if a module is run and there is multiple instances of a worksheet.

Is there any way to check if there are mutliple instances of a worksheet open?

Thanks, in advance, for your help.