Although I'm not 100% sure, I believe the queue app never calls a busy
agent.
The problem you might have is that your phone(s) have their ability to
receive multiple calls (on snom's it's called "Call Waiting
Indication")
enabled. If this feature is enabled, from the viewpoint of asterisk your
phone is able to receive a call even if you're busy handling one. To my
knowledge, the only way to prevent "busy" phones from being called by
the queue app is to dissable the "Call Waiting Indication" feature on
your agents/phones.
This "problem" of multiple exclusiveness between "Call Waiting
Indication" and call queues is more a fundamental rather than a
technical issue common to call center applications.
I hope this answers your question.