Every time I’m trying to return a Night Fae Anima Conductor quest, it’s a 50/50 chance that I get the “[Quest] failed: Duplicate item found.” error. I’ve tried reloading, disabling all addons, and even abandoning and re-doing the quest. Always the same. Apparently, this doesn’t apply only to the dailies, as right now I’m even unable to finish the “Gormling Piper: Tranquil Pools” world quest. So basically, since I started the court of night rep farm almost 2 weeks ago, I have to finish 2 long dailies and 1 world quest everyday, hoping that the game let’s me return at least one of them. The worst part is that I haven’t managed to find a single other person who has the same issue.
And I already have made an in-game bug report btw.
Sorry to hear about those issues that you are having there.
If you are receiving the message “duplicate item found”, that most likely means that you have a Unique item in your inventory or bank, that the quest is providing you again. You will basically need to find which quest item this is. Some extra information can be found in here, even though the article is for a completely different line of quests.
With regards to the Tranquil Pools, there is indeed an issue with this. The easiest way to resolve this problem is to head into the area of the quest, log out and then log back in. Make sure that you reset your interface before this as well. Some extra information can be located in here.