[Baypiggies] list question

[Vikram K]

i have this list:
x = ['cat','dog','dog']

i wish to identify the non-unique element in this list i.e. 'dog'. how do i
do this?

i did this:

>>> y = list(set(x))
>>> y
['dog', 'cat']

but now i don't know to proceed further. have tried the following:

>>> z = [i for i in x if i in y]
>>> z
['cat', 'dog', 'dog']

>>> z = x - y
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    z = x - y
TypeError: unsupported operand type(s) for -: 'list' and 'list'
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