I think this is a glitch. When you save Nadira after the Goblin attack in the village, I talked to her with my custom character and got the piece. When I exited the conversation, Nadira automaticaly talked to Shadow heart like it was scripted (I imagine I talked to her too quickly the first time and did not let the script play) and offered the same dialogue with the same road to the same reward. You can't get the reward twice fortunatly. Of course you usually have the same dialogue from scratch as it is planned for coop player so they can witnessed all dialogues but I don't think this auto dialogue should have happened if I had already talked to Nadira with my previous character and got the piece.
Have fun on the game. It's awesome.
