After checking (no I didn't spend a week doing it), I am fairly certain that Shannon = Kanon. Or more accurately, Kanon is a part of Shannon. Some evidence I dug up:
'I am one yet many'. One means Shannon. Many means Shannon, Kanon and Beatrice.
In episode 6, the couples must take part in a test, and only one pair can succeed. This is because one member of each pairing are actually the same person, you can't have Shannon loving 3 people at once. There is also that duel between Shannon and Kanon and how they are in the way of the other.
Will told Shannon to fetch Kanon in the chapel, but Shannon acted weird and implied Will should stop asking. Presumably this is to not give away that they are the same. In fact, I don't think they ever appeared together in the chapel part of episode 7.
The timing of Kanon's arrival to Rokkenjima, as mentioned a few posts ago.
When Genji was talking about the portrait, both Shannon and Kanon were shown, but he said 'you should try it too Shannon' as if Kanon wasn't here. Iirc it is the only time Kanon showed up in Clair's story.
I'm sure there are more hints but this is enough for now.