Comment 7 for bug 197781

A comment in nsXULDocument.cpp claims we don't support loading the same overlay twice into the same document, because it "doesn't make sense anyway":
http://bonsai.mozilla.org/cvsblame.cgi?file=/mozilla/content/xul/document/src/nsXULDocument.cpp&rev=1.752&mark=2658-2659#2658

In fact we do support it, both with document.loadOverlay (provided that there's a sufficient interval between the two loadOverlay calls) and normal overlays, as far as I can see. I don't see why we should forbid that.

So how should this work?