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Python | Find keys with duplicate values in dictionary
• Last Updated : 07 Aug, 2019

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'}
```

 `result ``=` `set``(chain.from_iterable(``         ``values ``for` `key, values ``in` `rev_dict.items()``         ``if` `len``(values) > ``1``))`
 `result ``=` `filter``(``lambda` `x: ``len``(x)>``1``, rev_dict.values())``print``(``list``(result))`