Dismantling an exotic item rewards one exotic shard. Infusing an exotic item costs one exotic shard.
If I infuse an exotic then dismantle it, do I receive one or two exotic shards? Furthermore, if I infuse twice, do I receive three exotic shards?
Always one. It 'costs' 1 shard to infuse, so to infuse you actually lose the shard. (for new players, like myself, who didn't have a ton of strange coins, this is a very big wall. I have blue items at 300 and exotics at 280-290 I can't use because I don't have enough shards to infuse them.)
Side note: You get less from infusing exotics also. 80% of the difference when infusing a legendary, but only 70% of the difference when infusing an exotic. Full 5 points if only 5 point difference with legendary, full 4 points if 4 point difference with exotic.