Yeah. My understanding is the message key used to decrypt first copy should be immediately deleted after decrypting it, so how can the second copy be decrypted successfully? Seems to fly in the face of forward secrecy unless it's purely a client side bug.

Bad encrypted message, if I recall correctly, means either the MAC check failed or the protobuf payload inside didn't deserialize correctly.

I don't know.. I'm not setup to debug it on my regular phone with a production signal apk installed.