I think it is because the when its proc spell crits, it also triggered another thrust of tattered veils. I believes that's the reason why it triggered twice, I think if there is a low deflection high hp meat dummy we can test the guess And if the enemies's deflection is low enough it will chain the critical attack, which will kill the mob in one shot

Don't think so. It always fires twice. Never once, never thrice.

 

Nice phrase for a chant by the way.

 

It used to only fire once. So either this is a bug and related to others of the same kind (Stormcaller's Returning Storm hits enemy and yourself, Redeemer's Spell dito and so on) - or it's intended. Since this is Oopsidian I think it's a bug.