Identify duplicate values in a list in Python

Is it possible to get which values are duplicates in a list using python?

I have a list of items:

    mylist = [20, 30, 25, 20]

I know the best way of removing the duplicates is set(mylist), but is it possible to know what values are being duplicated? As you can see, in this list the duplicates are the first and last values. [0, 3].

Is it possible to get this result or something similar in python? I'm trying to avoid making a ridiculously big if elif conditional statement.

Solution 1:

These answers are O(n), so a little more code than using mylist.count() but much more efficient as mylist gets longer

If you just want to know the duplicates, use collections.Counter

from collections import Counter
mylist = [20, 30, 25, 20]
[k for k,v in Counter(mylist).items() if v>1]

If you need to know the indices,

from collections import defaultdict
D = defaultdict(list)
for i,item in enumerate(mylist):
    D[item].append(i)
D = {k:v for k,v in D.items() if len(v)>1}

Solution 2:

Here's a list comprehension that does what you want. As @Codemonkey says, the list starts at index 0, so the indices of the duplicates are 0 and 3.

>>> [i for i, x in enumerate(mylist) if mylist.count(x) > 1]
[0, 3]

Solution 3:

The following list comprehension will yield the duplicate values:

[x for x in mylist if mylist.count(x) >= 2]