I was able to pick up a copy of the map fragment requiring ascended rocket kills on Nessus from Petra before completing a nightfall, completed it, and after I completed a nightfall I recieved another copy of the same map fragment, and out of curiousity I completed it and it turns out both could count towards the overall quest requiring four different map fragments being decoded. I feel like this could be an exploit and if it is not working as intended, I am more than willing to re-do the whole quest from the beginning in order to complete it as intended by the designers and developers. I am not a fan of exploits being in games and wanted to let you all know that this could be a hole to patch. Thanks for all you do!