Excel is considering that a file is already opened and it has the same name as the file you are trying to open. Therefore, change the name of the file you are opening. This should help.

How to Open Excel 2007 Files As Separate Windows. If you work with multiple Excel files over the course of a workday, and especially if you use a dual-monitor setup, you might wonder how you can ...

Q: When I open two Excel files in two instances of Excel and display them on two monitors, I find that I cannot then copy formulas back and forth between the files—only the formula’s calculated value ...

How to Open an XLS File in Excel. XLS files are Excel workbook files created in Excel 97 or Excel 2003. The 2007, 2010 and 2013 versions of the program use the XLSX format by default to save ...

If you cannot open an Excel file and see Excel cannot open the file because the file format or extension is not valid error, see this post.