Open unopened file

At a certain stage, my program prompts user to open file "FileToOpen" where "FileToOpen" is a string variable assigned to the name of a certain file. In order to get to this point, I use the command: FileToOpen = Application.GetOpenFilename(<various filters>)

If FileToOpen is already open, I get a system alert. Rather than suppress the alert, I want my program to do something like this: