Python 3.5.2 (default, Sep 30 2016, 01:32:24)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-17)] on linux
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>>> l=[2,2,3,4,5,6,7,7,7,7]
>>> subl=[i for i in l if l.count(i)>1]
>>> subl
[2, 2, 7, 7, 7, 7]
>>> set(subl)
{2, 7}