[yangxu]

Spoiler:

I don't get how the servants could mistake an imposter of Kanon for the real Kanon. Let's say the imposter dressed exactly like the real Kanon, his face would have to be nearly identical to the real one to fool 4 people at the same time. Unless of course, Kanon is simply a "codename" or "title" that can be adapted by anyone who has the proper "equipments", like what Rias proposed about the new Kinzo.

So there are three possibilities:

1. Kanon had a twin
2. An imposter adopted the title "Kanon" and attempted to murder everyone
3. The real Kanon faked death and attempted to kill everyone afterwards