We see this assert failing occasionally with Shenandoah: https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shared/stringdedup/stringDedupTable.cpp#L626

Shenandoah (and ZGC) both process weak references concurrently. I believe the weak reference processing threads may also run concurrently with the string deduplication thread. In the cases I've seen, the `found` referent was not marked after final mark and so may not be 'resurrected' here during deduplication.