You can tell for certain, if you leave the child selected when they pick up the collectible. The way to tell whether or not it's a duplicate collectible is to check the action field. If it says, "Collecting an item for the scientists" then the collectible is a duplicate. If it says what kind of collectible they picked up (e.g. "Collecting a rare fish scale!"), then it's a new one, and they will take it to the statue.