Does this only happen with certain workbooks?

And if you look at the titlebar, do you see things like:

book1.xls:1
and
book1.xls:2

If you do, then it's not really two instances of excel and it's not even two
versions of your workbook. It's two (at least) windows looking at the same
workbook.

You can close one of these windows by clicking on the window you don't want then
hitting ctrl-w.

Debra Dalgleish has some nice pictures explaining another way to close the
window at:
http://www.contextures.com/xlfaqApp.html#TwoWindows

Remember to save your workbook the way you want it--or you'll be doing this
again.

Pinnacle wrote:

When opening an excel spreadsheet either from Excel or explorer, excel
automatically opens another instance (workbook1) of excel.

--

Dave Peterson