I'm not sure if this is an Apple-Java problem or if it is actually related to Stendhal as I don't have an Apple to test this on. But it was reported to me that only a single instance of the client can be opened on Apple computers.
How the client is started? It's possible that the OS tries to do something "smart". Stendhal does no checking whatsoever to see if there are other instances.
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here are some explanations and what to do against it.
that you need on mac a terminal isnt userfriendly... lol
So it sounds like the simple answer to this would be to use/include a shell script for Apple uses that invokes the "java -jar" command. I'll have to test this out on my friends Mac.
Closing as the bug is not related to Stendhal, but OS X.