I've heard you have to put all of your 'collected' exotics on you before going to Xur or the Fated Engram won't give you something new. I did this just in case (put everything 'on' before I went) and I got a Jade Rabbit (something new). I think it's stupid that the game doesn't read your 'collection', but until my collection completely fills my inventory, I can equip what I have to work around this. And yes, I do not understand how they screw this stuff up so badly...