How to Find Duplicates in a List - Python

How to Find Duplicates in a List - Python

Last Updated : 27 Nov, 2024

Finding duplicates in a list is a common task in programming. In Python, there are several ways to do this. Let’s explore the efficient methods to find duplicates.

Using a Set (Most Efficient for Large Lists)

Set() method is used to set a track seen elements and helps to identify duplicates.

Python

a = [1, 2, 3, 4, 5, 2, 6, 3]

# A set to keep track of elements that have been seen
seen = set()
# A list to store duplicates found in the input list
duplicates = []

# Iterate over each element in the list
for i in a:
    if i in seen:
        duplicates.append(i)
    else:
        seen.add(i)

print(duplicates)

Output

[2, 3]

There are more approaches to Find Duplicates in a List :

Table of Content

Using collection.counter
Using List Comprehension

Using collection.counter

Using collection.counter is efficient as in this Counter class from the collections module is greater for counting occurrences of each element .

Python

from collections import Counter

a = [1, 2, 3, 4, 5, 2, 6, 3]

# Use Counter to count 
#the occurrences of each element in the list
counts = Counter(a)
duplicates = [item for item, count in counts.items() if count > 1]
print(duplicates)

Output

[2, 3]

Using List Comprehension

List comprehension allows us to write more compact code. We then use a list comprehension to find items that appear more than once and store them in the duplicates list. Here’s how we can find duplicates using list comprehension.

Python

a = [1, 2, 3, 4, 5, 2, 6, 3]

# Create a list of duplicates using list comprehension
duplicates = [i for i in set(a) if a.count(i) > 1]
print(duplicates)

Output

[2, 3]

How to Find Duplicates in a List - Python

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