Dung Nguyen and I just noticed that double-clicking on the application (or on the jar file) doesn’t bring up a new instance on his Mac (OS X 10.6). It does so on mine (OS X 10.4).
In the end, we started DrJava from the command line, and that worked. I would still like to find out what the difference is. Does OS X 10.5 already behave like this? Is there some setting that governs this behavior?
If you use a Mac, could you please try to start more than one instance of DrJava from the Finder and report back?