Vaughan,
Well, this is weird. I had been doing as you suggested, i.e. in a button was the perform script command, executing the command: "Open file ["Volunteers"]. And the result was alway a clone when the file was already open, i.e. Volunteer 2, Volunteers 3, etc.
But I just tried it again and I get the same results I used to get in FileMaker 5, i.e. it opens once and has no effect if it is already open.
Very strange.