Help improve 'Non Unique Elements'

Hi there! We urgently need your help. One of our first missions on the home island, called Non Unique Elements, needs a little advanced-coder-love. We want to give people who are new to coding and new to CheckiO the possibility to browse through some top notch solutions... from you guys!

We hope you can help us wow novice coders, by publishing your solution: http://www.checkio.org/mission/task/info/non-unique-elements/

Thank you!!