Comment 7 for bug 665452

i modified ZODB/DB.py:

    def save_oid(self, oid):
        if oid in self._saved_oids:
            raise ValueError("Duplicate saved object ids.")
+ return
        if len(self._saved_oids) < self._max_saved_oids:
            self._saved_oids.add(oid)

I'll try packing it again today night, when traffic is less on our site. right now this nasty bug leaves us with a couple of angry customers, as the errors don't go away until we restart the appserver. If you have any suggestions as to what code paths i might try to deactivate for debugging, i'm happy to help out. unfortunately i still cannot make a small reproduceable test case from this.
it strongly looks like some kind of cache corruption though.