In Python, to find all duplicate elements of a list, we have to first understand the mathematical concept of a set. A set contains only a list of distinct elements. Python provides the same concept with the same name, set.
In the example below, we will exploit the property of a set. We will read through a list of numbers and put each number into a set called seen. Since seen is a set, any duplicate numbers that we are trying to add are ignored. As result, seen will only hold a list of unique numbers. However, the numbers inside a set are stored in order. To keep the ordering and still get a list of unique numbers, we simply introduce another variable called unique. Numbers are added in unique variable at time that we add numbers in the seen variable.
my_list = [ 1, 5, 2, 4, 2, 5 ] seen = set() unique =  duplicate =  for x in my_list: if x not in seen: unique.append(x) seen.add(x) else: duplicate.append(x) print(duplicate, " are duplicates.") print(unique, " are unique.")
[2, 5] are duplicates. [1, 5, 2, 4] are unique.