def checkio(data):
        #print ('input', data)
        for i in data:
            #print ('i', i)
            if data.count(i) == 1: data.remove(i); #print ('count=', data.count(i), '-del')
        #print ('output', data)
        return data

This code run in strange ways: some times cycle for not run properly and drop some counts.

Here output for this code:

    >>>checkio([1,2,3,2,1])
    i 1
    i 2
    i 3
    count= 0 -del
    i 1
    <<< [1, 2, 2, 1] # ALL OK, NO BUG


    >>>checkio([1,2,3,4,5])
    i 1
    count= 0 -del
    i 3
    count= 0 -del
    i 5
    count= 0 -del
    <<< [2, 4] #??? BUG on even counts


    >>>checkio([1,2,3,4,5,6,1])
    i 1
    i 2
    count= 0 -del
    i 4
    count= 0 -del
    i 6
    count= 0 -del
    <<< [1, 3, 5, 1]   #??? BUG odd counts

From: https://py.checkio.org/mission/non-unique-elements/solve/

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