Find Duplicate Elements in a Python List

Identifies and returns duplicate elements from a Python list using sets for efficient membership tests.

Easy Python 3.9+ Aug 9, 2026 Lists & loops 13 views 0 copies

duplicates sets list

Python code

13 lines

Python 3.9+

def find_duplicates(lst):
    seen = set()
    duplicates = set()
    for item in lst:
        if item in seen:
            duplicates.add(item)
        else:
            seen.add(item)
    return list(duplicates)

if __name__ == "__main__":
    sample = [1, 2, 3, 2, 4, 1, 5, 3]
    print(find_duplicates(sample))

Output

stdout

[1, 3, 2]

How it works

This function uses two sets: seen tracks items already encountered, and duplicates stores items that appear more than once. For each element, if it's already in seen, it's added to duplicates; otherwise, it's added to seen. Since sets ignore ordering, the output order may vary between runs. The solution runs in O(n) time on average.

Common mistakes

Using a list to track seen items, which makes the function O(n^2).
Not converting the result to a list, leading to an unordered set output.
Counting occurrences and then filtering, which is less efficient for large lists.

Variations

Use a dictionary to count occurrences and then filter keys with count > 1.
Use a list comprehension with counting but note it is slower for large data.

Real-world use cases

Detecting duplicate user IDs in a dataset before syncing to a CRM.
Finding repeated error codes in a log file to highlight frequent issues.
Identifying duplicate SKU entries in an inventory CSV before import.

Find Duplicate Elements in a Python List

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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How it works

Common mistakes

Variations

Real-world use cases

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