Via a qerk, the blue scribes seem to have a way of manifesting a power that has already been attempted (and manifested) in the psychic phase.
RAW, states that a psykers cannot "attempt" a power that has already been "attempted" in the psychic phase, Blue scribes "automatically manifest" a random power, so by not "attempting" it would be exempt from the rule. - FAQ confirms this.
Some of the powers state "add +1" or "add 1" use words like "may re-roll a single".
Which one's stack or don't stack. I've checked the FAQ's and there's nothing there qualifying this, and which powers could stack.