How do I find the duplicates in a list and create another list with them?

In Python, one way to find duplicates in a list and create another list with them is to use a for loop and an if statement. Here is an example code snippet:

Find duplicates in a list and create another list with them in Python

This code first creates an empty list called "duplicate_list" to store the duplicates. Then, it iterates through the elements of the "original_list" using a for loop. For each element, it checks if the count of that element in the "original_list" is greater than 1 (i.e., if it is a duplicate). If it is, it then checks if the element is already in the "duplicate_list", and if not, it adds it to the "duplicate_list". Finally, it prints out the "duplicate_list".

Alternatively, you can use a list comprehension to achieve the same logic more concisely. Note that this approach returns each duplicate multiple times:

Find duplicates in a list and create another list with them in Python using list comprehension

This code snippet uses list comprehension and the count() function to filter elements that appear more than once. Like the first approach, it has O(n²) time complexity because count() scans the list for every item. For larger datasets, or when you need unique duplicates only, collections.Counter is more efficient:

Find duplicates in a list using collections.Counter

from collections import Counter

original_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 2, 4, 6, 8]
counts = Counter(original_list)
duplicate_list = [item for item, count in counts.items() if count > 1]

print(duplicate_list)

This approach counts all elements in a single pass (O(n) time complexity) and filters those with a count greater than 1, ensuring each duplicate appears only once in the result.