If you have 50 different plug types, appliances wouldn't be available and would be very
expensive. But once an electric outlet becomes standardized, many companies can design
appliances, and competition ensues, creating variety and better prices for consumers.
-- Bill Gates

Non-unique Elements
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You are given a non-empty sequence of integers. For this task, you should return Iterable consisting of only the non-unique elements from the initial sequence. To do so you will need to remove all unique elements (elements which are contained in a given sequence only once). When solving this task, do not change the order of the elements. Example: in [1, 2, 3,...
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