I'd have thought declaring previous applications shows honesty and a passion for aviation. During a recent interview for a funded cadet programme I was asked if I had applied elsewhere and whether I would continue to apply in the future. It was clear that answering "no" to either question would have resulted in a follow up of "why not?".
Additionally, anyone who made the AoN stage elsewhere would presumably have some sort of profile with AoN, so I'd assume they'd know if people answered dishonestly and got to that stage with Jet2.