For a popular singing contest that is broadcast on TV the organizers have hired a company that will develop a system with which one can vote for the different contestants via text message. According to the rules of the competition, it is only possible to vote once with every mobile phone. Only the first vote that was sent from a certain telephone is saved in the competition's database. At least, that was the intention. Because of a bug in the software (or maybe even malicious intent), multiple votes per telephone were registered. can you find out which telephones have cast multiple votes?
Write a function doubles to which a list of strings or integers must be given as a parameter. This parameter could represent, for example, a list of telephone numbers that have cast a registered vote for a contestant of the singing contest. As a result, the function must print a list that contains all elements that occur more than once in the original list (in this case: the telephone numbers that have cast more than one vote). These elements must be in an increasing order in the list that was printed as result.
>>> doubles([3, 9, 4, 3, 8, 7, 3, 4, 2])
[3, 4]
>>> doubles([1, 2, 3, 4, 5, 6, 7, 8, 9])
[]
>>> doubles([8, 6, 9, 5, 7, 4, 8, 3])
[8]
>>> doubles(['0476-987394', '0498-837493', '0476-987394'])
['0476-987394']