It turns out to be rather difficult to "know" under which context the portal is running when you're not invoked from a servlet request.
As the DeployPortletAppEventListener is running in its own thread, and can invoke a deployment when not one request has been send of to the portal,
this becomes very difficult to handle automatically.
And then, what if the war contains a context.xml (Tomcat) redefining its context? Should we start checking that too?
And finally, a user can also accidentally drop a war directly in the (Tomcat) webapps folder, certainly leading to disaster.
In short, I don't think this can really be handled/fixed 100%.
Furthermore, in my opinion, this is a user error which hardly will ever occur and certainly not twice by the same user for sure
So, I'm going to classify this issue as "Won't Fix" for now.
If anyone thinks differently, then just reopen it again.