First solution in Clear category for Non-unique Elements by Mateusz_Tomkowiak
#Your optional code here
#You can import some modules or create additional functions
#Your code here
#It's main function. Don't remove this function
#It's used for auto-testing and must return a result for check.
arr = 
for item in data:
if(data.count(item) != 1):
#You can use list.count(element) method for counting.
#Create new list with non-unique elements
#Loop over original list
Oct. 21, 2016