Python | Find keys with duplicate values in dictionary

Given a dictionary, the task is to find keys with duplicate values. Let’s discuss a few methods for the same.

Method #1: Using Naive approach
In this method first, we convert dictionary values to keys with the inverse mapping and then find the duplicate keys

# Python code to demonstrate
# finding duplicate values from a dictionary

# initialising dictionary
ini_dict = {'a':1, 'b':2, 'c':3, 'd':2}

# printing initial_dictionary
print("initial_dictionary", str(ini_dict))

# finding duplicate values
# from dictionary
# using a naive approach
rev_dict = {}

for key, value in ini_dict.items():
    rev_dict.setdefault(value, set()).add(key)

result = [key for key, values in rev_dict.items()
                              if len(values) > 1]

# printing result
print("duplicate values", str(result))

Output:

initial_dictionary {'c': 3, 'b': 2, 'd': 2, 'a': 1}
duplicate values [2]

Method #2: Using flipping dictionary

# Python code to demonstrate
# finding duplicate values from dictionary

# initialising dictionary
ini_dict = {'a':1, 'b':2, 'c':3, 'd':2}

# printing initial_dictionary
print("initial_dictionary", str(ini_dict))

# finding duplicate values
# from dictionary using flip
flipped = {}

for key, value in ini_dict.items():
    if value not in flipped:
        flipped[value] = [key]
    else:
        flipped[value].append(key)

# printing result
print("final_dictionary", str(flipped))

Output:

initial_dictionary {'a': 1, 'c': 3, 'd': 2, 'b': 2}
final_dictionary {1: ['a'], 2: ['d', 'b'], 3: ['c']}

Method #3: Using chain and set

Suppose you need to find keys having duplicate values.

# Python code to demonstrate
# finding duplicate values from dictionary
from itertools import chain

# initialising dictionary
ini_dict = {'a':1, 'b':2, 'c':3, 'd':2}

# printing initial_dictionary
print("initial_dictionary", str(ini_dict))

# finding duplicate values
# from dictionary using set
rev_dict = {}
for key, value in ini_dict.items():
    rev_dict.setdefault(value, set()).add(key)


result = set(chain.from_iterable(
         values for key, values in rev_dict.items()
         if len(values) > 1))

# printing result
print("resultant key", str(result))

Output:

initial_dictionary {'b': 2, 'd': 2, 'c': 3, 'a': 1}
resultant key {'d', 'b'}

Instead of

result = set(chain.from_iterable(
         values for key, values in rev_dict.items()
         if len(values) > 1))

Use this:

result = filter(lambda x: len(x)>1, rev_dict.values())
print(list(result))