How to find duplicate elements in array using for loop in Python?

Question

Just for information, In python 2.7+, we can use Counter

import collections

x=[1, 2, 3, 5, 6, 7, 5, 2]

>>> x
[1, 2, 3, 5, 6, 7, 5, 2]

>>> y=collections.Counter(x)
>>> y
Counter({2: 2, 5: 2, 1: 1, 3: 1, 6: 1, 7: 1})

Unique List

>>> list(y)
[1, 2, 3, 5, 6, 7]

Items found more than 1 time

>>> [i for i in y if y[i]>1]
[2, 5]

Items found only one time

>>> [i for i in y if y[i]==1]
[1, 3, 6, 7]

Use the in operator instead of calling __contains__ directly.

What you have almost works (but is O(n**2)):

for i in xrange(len(list_a)):
  for j in xrange(i + 1, len(list_a)):
    if list_a[i] == list_a[j]:
      print "duplicate:", list_a[i]

But it's far easier to use a set (roughly O(n) due to the hash table):

seen = set()
for n in list_a:
  if n in seen:
    print "duplicate:", n
  else:
    seen.add(n)

Or a dict, if you want to track locations of duplicates (also O(n)):

import collections
items = collections.defaultdict(list)
for i, item in enumerate(list_a):
  items[item].append(i)
for item, locs in items.iteritems():
  if len(locs) > 1:
    print "duplicates of", item, "at", locs

Or even just detect a duplicate somewhere (also O(n)):

if len(set(list_a)) != len(list_a):
  print "duplicate"

You could always use a list comprehension:

dups = [x for x in list_a if list_a.count(x) > 1]

Before Python 2.3, use dict() :

>>> lst = [1, 2, 3, 5, 6, 7, 5, 2]
>>> stats = {}
>>> for x in lst : # count occurrences of each letter:
...     stats[x] = stats.get(x, 0) + 1 
>>> print stats
{1: 1, 2: 2, 3: 1, 5: 2, 6: 1, 7: 1} # filter letters appearing more than once:
>>> duplicates = [dup for (dup, i) in stats.items() if i > 1] 
>>> print duplicates

So a function :

def getDuplicates(iterable):
    """
       Take an iterable and return a generator yielding its duplicate items.
       Items must be hashable.

       e.g :

       >>> sorted(list(getDuplicates([1, 2, 3, 5, 6, 7, 5, 2])))
       [2, 5]
    """
    stats = {}
    for x in iterable : 
        stats[x] = stats.get(x, 0) + 1
    return (dup for (dup, i) in stats.items() if i > 1)

With Python 2.3 comes set(), and it's even a built-in after than :

def getDuplicates(iterable):
    """
       Take an iterable and return a generator yielding its duplicate items.
       Items must be hashable.

       e.g :

       >>> sorted(list(getDuplicates([1, 2, 3, 5, 6, 7, 5, 2])))
       [2, 5]
    """
    try: # try using built-in set
        found = set() 
    except NameError: # fallback on the sets module
        from sets import Set
        found = Set()

    for x in iterable:
        if x in found : # set is a collection that can't contain duplicate
            yield x
        found.add(x) # duplicate won't be added anyway

With Python 2.7 and above, you have the collections module providing the very same function than the dict one, and we can make it shorter (and faster, it's probably C under the hood) than solution 1 :

import collections

def getDuplicates(iterable):
    """
       Take an iterable and return a generator yielding its duplicate items.
       Items must be hashable.

       e.g :

       >>> sorted(list(getDuplicates([1, 2, 3, 5, 6, 7, 5, 2])))
       [2, 5]
    """
    return (dup for (dup, i) in collections.counter(iterable).items() if i > 1)

I'd stick with solution 2.