I had exactly the same today. I assumed that it was trying to unlock a specific character, and it was one I already had, but after grinding deaths for an hour, it would have been nice to not get a quest that wouldn't give me something.
It will give you the value of the unlocked character if you get it a different way... It will say "Already Unlocked" and give you coins and pops instead.
As a note, resurrecting 5 times is really hard. I had to down-gear enough to die and need to be resurrected, but not so much that I couldn't complete the level. I think I ran the same level for an hour trying to complete the quest.
