Thread:DanteVin/@comment-3236835-20150827023543/@comment-33460495-20150827050632

Yes, and unlike Order of Ecclesia, you can only have 5 at a given time, as it is the maximum number of quest slots in Portrait of Ruin. Once you have 5, you cannot accept new quests. You must then clear a quest to free the slot, allowing you to take another one.

In my first playthrough, the "Poison vs. Poison" was stuck in its slot because I already sold the Long Sword, not knowing that it is a requirement for the quest. Therefore I find it impossible to complete until I started a New Game Plus.

And there is also a Quest Item Duplication Trick from this guide, you might want to check it out.