Find Duplicates in Python List Fast

You can use sets to quickly find duplicate items.

Option 1: Method one involves using a for loop and a collection to find duplicate items.

def find_duplicates(lst):
    duplicates = set()
    for item in lst:
        if lst.count(item) > 1:
            duplicates.add(item)
    return duplicates

lst = [1, 2, 3, 4, 4, 5, 6, 6, 7]
print(find_duplicates(lst))

Output:

{4, 6}

Option 1:
Method 2: Find duplicate items using list comprehension and sets.

def find_duplicates(lst):
    return set([item for item in lst if lst.count(item) > 1])

lst = [1, 2, 3, 4, 4, 5, 6, 6, 7]
print(find_duplicates(lst))

Output:

{4, 6}