Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int index=0;
    for(int iter = 0;iter<n;iter++)
    {
    for(int ind = iter+1;ind<n;ind++)
    {
      if(arr[iter]==arr[ind])  
        return arr[ind];
    }
    }
    return -1;
}

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

    int arr[100];
    for (int i = 0; i < n; i++) scanf("%d", &arr[i]);

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

Solving Approach