This glitch happens when you are already holding one of the three collectables (the jewel, the coin and the fish) and try to get more. The collectable you are holding is the only type of collectable you can duplicate however, so you can't get guaranteed jewels while holding a coin
If you bring a fish you've caught from a different port to a port with 1 fish already in a net, casting an empty net doesn't guarantee another fish catch. This prooves that you can only catch a guaranteed fish if there aren't any fish (randomly caught OR guaranteed) already on the same port.