Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>
int find_duplicate(int arr[], int n) 
{
    int cnt=0;
    for(int i=0;i<n;i++)
    {
        cnt=0;
        for(int j=i+1;j<n;j++)
        {
            if(arr[i]==arr[j])
            {
                cnt++;
            }
        }
        if(cnt==1)
        {
            return arr[i];
        }
    }
    return -1;
}

int main() 
{
    int n;
    scanf("%d", &n);

    int arr[n];
    for (int i = 0; i < n; i++) 
        scanf("%d", &arr[i]);

    int result = find_duplicate(arr, n);
    printf("%d", result);
    return 0;
}

Solving Approach