Update: using the Admin Console to deploy the EAR from its original location, in the Eclipse project's build-target output folder, works just fine. Deploys without incident and only once.
you should ask this in the JBoss AS Users forum: http://www.jboss.org/index.html?module=bb&op=viewforum&f=287, as this forum is only for Microcontainer users.
Do you have Seam in your app? I remember having read about some issues like this with Seam.