Nevermind ... its just been too long since I had the dupplicate items issues. It happens when you have a placed NPC in an area and want to add it to the pallette that items in its inventory get duplicate blueprints added to the pallettes. After some testing, it also happens with placeables too.
thought so, but you said "export"
easy to workaroud however
just add to palette and exit window (X in right top corner) for any item you dont want to make duplicate