When I open an Excel spread sheet I've previously created, I get a
strange message.

"A document with the name "My Spreadsheet.xls" is already open. You
cannot open two documents with the same name, even if the documents are
in different folders.
To open the second document, either close the document that's currently
open, or rename one of the documents."

In fact no other spreadsheet is open. I click OK and I'm into the
spreadsheet.

Any suggestions? I'm able to use the program, access my spreadsheets,
but it's annoying.

And I'd prefer not to let this go until one day I CAN'T use my
spreadsheets.