The class ability Canny Defense is granted by the Kensai Archetype of the Magus class, and states that it works the same as the Duelist ability of the same name.
So if a character goes into Duelist, do the levels stack or is the bonus applied again?
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The rules are silent. I would say that the level stack.
Notice that canny defense work with a light armor and it is possible to get armors with a very low spell failure chance.
A mithral shirt has no no armor check penalty so you can use one even without the light armor proficiency.
I don't like adding oriental armor in the Inner Sea setting (barring special circumstances) but a Haramaki or a Silken ceremonial armor have a AC value of +1, no armor check penalty or spell failure and can be enhanced. Most of the time they would be better (price wise) than bracers of armor giving the same AC (armour+enhancment).
Arcane Armor Training require to be proficient with light armor and a swift action, so a relatively high feat investment, but it can be a good idea.