How to check if array contains a duplicate number using C#?

Checking if an array contains duplicate numbers is a common programming task in C#. There are multiple approaches to solve this problem, ranging from simple loops to LINQ methods and hash-based techniques.

Using Dictionary to Count Occurrences

The dictionary approach counts the frequency of each element in the array. Any element with a count greater than 1 is a duplicate −

using System;
using System.Collections.Generic;

namespace Demo {
    public class Program {
        public static void Main(string[] args) {
            int[] arr = {87, 55, 23, 87, 45, 23, 98};
            var dict = new Dictionary<int, int>();
            
            foreach(var num in arr) {
                if (dict.ContainsKey(num))
                    dict[num]++;
                else
                    dict[num] = 1;
            }
            
            Console.WriteLine("Number occurrences:");
            bool hasDuplicates = false;
            foreach(var val in dict) {
                Console.WriteLine("{0} occurred {1} times", val.Key, val.Value);
                if (val.Value > 1) hasDuplicates = true;
            }
            
            Console.WriteLine("\nArray contains duplicates: " + hasDuplicates);
        }
    }
}

The output of the above code is −

Number occurrences:
87 occurred 2 times
55 occurred 1 times
23 occurred 2 times
45 occurred 1 times
98 occurred 1 times

Array contains duplicates: True

Using HashSet for Efficient Duplicate Detection

A HashSet provides a more efficient way to check for duplicates by attempting to add each element. If an element already exists, Add() returns false −

using System;
using System.Collections.Generic;

namespace Demo {
    public class Program {
        public static void Main(string[] args) {
            int[] arr = {87, 55, 23, 87, 45, 23, 98};
            var hashSet = new HashSet<int>();
            
            bool hasDuplicates = false;
            foreach(int num in arr) {
                if (!hashSet.Add(num)) {
                    Console.WriteLine("Duplicate found: " + num);
                    hasDuplicates = true;
                }
            }
            
            Console.WriteLine("Array contains duplicates: " + hasDuplicates);
        }
    }
}

The output of the above code is −

Duplicate found: 87
Duplicate found: 23
Array contains duplicates: True

Using LINQ GroupBy Method

LINQ provides a concise way to group elements by their value and identify duplicates −

using System;
using System.Linq;

namespace Demo {
    public class Program {
        public static void Main(string[] args) {
            int[] arr = {87, 55, 23, 87, 45, 23, 98};
            
            var duplicates = arr.GroupBy(x => x)
                               .Where(group => group.Count() > 1)
                               .Select(group => group.Key);
            
            Console.WriteLine("Duplicate numbers:");
            foreach(var duplicate in duplicates) {
                Console.WriteLine(duplicate);
            }
            
            bool hasDuplicates = duplicates.Any();
            Console.WriteLine("\nArray contains duplicates: " + hasDuplicates);
        }
    }
}

The output of the above code is −

Duplicate numbers:
87
23

Array contains duplicates: True

Comparison of Methods

Conclusion

There are multiple efficient ways to check for duplicates in an array using C#. The HashSet approach is most efficient for simple duplicate detection, while the Dictionary method is best when you need to count occurrences. LINQ provides a concise, readable solution for functional programming enthusiasts.