How do I find the duplicates in a list and create

Question

How can I find the duplicates in a Python list and create another list of the duplicates? The list only contains integers.

Answer 1

I would do this with pandas, because I use pandas a lot

import pandas as pd
a = [1,2,3,3,3,4,5,6,6,7]
vc = pd.Series(a).value_counts()
vc[vc > 1].index.tolist()

Gives

[3,6]

Probably isn't very efficient, but it sure is less code than a lot of the other answers, so I thought I would contribute

Answer 2

We can use itertools.groupby in order to find all the items that have dups:

from itertools import groupby

myList  = [2, 4, 6, 8, 4, 6, 12]
# when the list is sorted, groupby groups by consecutive elements which are similar
for x, y in groupby(sorted(myList)):
    #  list(y) returns all the occurences of item x
    if len(list(y)) > 1:
        print x  

The output will be:

4
6

Answer 3

def removeduplicates(a):
  seen = set()

  for i in a:
    if i not in seen:
      seen.add(i)
  return seen 

print(removeduplicates([1,1,2,2]))

Answer 4

collections.Counter is new in python 2.7:

Python 2.5.4 (r254:67916, May 31 2010, 15:03:39) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
a = [1,2,3,2,1,5,6,5,5,5]
import collections
print [x for x, y in collections.Counter(a).items() if y > 1]
Type "help", "copyright", "credits" or "license" for more information.
  File "", line 1, in 
AttributeError: 'module' object has no attribute 'Counter'
>>> 

In an earlier version you can use a conventional dict instead:

a = [1,2,3,2,1,5,6,5,5,5]
d = {}
for elem in a:
    if elem in d:
        d[elem] += 1
    else:
        d[elem] = 1

print [x for x, y in d.items() if y > 1]

Answer 5

this is the way I had to do it because I challenged myself not to use other methods:

def dupList(oldlist):
    if type(oldlist)==type((2,2)):
        oldlist=[x for x in oldlist]
    newList=[]
    newList=newList+oldlist
    oldlist=oldlist
    forbidden=[]
    checkPoint=0
    for i in range(len(oldlist)):
        #print 'start i', i
        if i in forbidden:
            continue
        else:
            for j in range(len(oldlist)):
                #print 'start j', j
                if j in forbidden:
                    continue
                else:
                    #print 'after Else'
                    if i!=j: 
                        #print 'i,j', i,j
                        #print oldlist
                        #print newList
                        if oldlist[j]==oldlist[i]:
                            #print 'oldlist[i],oldlist[j]', oldlist[i],oldlist[j]
                            forbidden.append(j)
                            #print 'forbidden', forbidden
                            del newList[j-checkPoint]
                            #print newList
                            checkPoint=checkPoint+1
    return newList

so your sample works as:

>>>a = [1,2,3,3,3,4,5,6,6,7]
>>>dupList(a)
[1, 2, 3, 4, 5, 6, 7]

Answer 6

Using pandas:

>>> import pandas as pd
>>> a = [1, 2, 1, 3, 3, 3, 0]
>>> pd.Series(a)[pd.Series(a).duplicated()].values
array([1, 3, 3])