How to find duplicate entry in Solidity array

A common method for finding duplicate data in Solidity arrays is to use a hash map (typically implemented via mapping). This approach enables us to detect duplicate elements with high efficiency (with an average time complexity close to O(n)). I will demonstrate a simple example where we use a mapping to track the occurrence count of each element in the array to identify duplicates.

solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract DuplicateFinder {
    // Function to find duplicate elements in an array
    function findDuplicates(uint[] memory arr) public pure returns (uint[] memory) {
        // Used to track the occurrence count of each element
        mapping(uint => uint) count;
        // Dynamic array to store the results
        uint[] memory duplicates;
        
        // Iterate through the input array, updating the count in the mapping
        for (uint i = 0; i < arr.length; i++) {
            count[arr[i]]++;
            // If the count reaches 2, it indicates a duplicate
            if (count[arr[i]] == 2) {
                // Add the duplicate element to the results array
                duplicates.push(arr[i]);
            }
        }
        
        return duplicates;
    }
}

Analysis:

1. Initialization: We use mapping(uint => uint) to track the occurrence count of each element in the array. Additionally, we create a dynamic array duplicates to store the identified duplicate elements.
2. Traversing the array: We iterate through the input array, incrementing the count in the mapping for each encountered element.
3. Detecting duplicates: After updating the count, we check if it reaches 2; if so, it indicates that the element has appeared before, thus being a duplicate. We then add it to the duplicates array.
4. Returning results: The function finally returns the array containing all identified duplicate elements.

Notes:

• In actual contracts, additional considerations are needed, such as the function's visibility (whether it should be public or external), whether it should be exposed externally, and call permissions.
• Furthermore, this method only records the first occurrence of duplicates; if an element appears multiple times (more than twice) in the array, the above implementation will not add it again to the result array. This can be adjusted based on specific requirements.

This is one method for finding duplicate data in Solidity arrays along with its implementation. In practical applications, this method is typically efficient and easy to implement.