Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>
//#include <stdbool.h>

int find_duplicate(int arr[], int n) {
    int i;
    i = 0;

    while(i < n)
    {
        int j = i + 1;
        int tmp = arr[j];

        while(j--)
        {
            if(arr[j] == tmp)
                return tmp;
            
        }
        i++;
    }
     
    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