Find Duplicate in Range 0 and n-1

#include <stdio.h>


/*
 * 0 1 2 3 4 5 6
 * 6 5 4 3 2 1 3
 
*/
int abs(int a) { return a > 0 ? a : (-a); }
int find_duplicate(int arr[], int n) {
    // Your logic here
    for (int i = 0; i < n; ++i) {
        for (int j = i + 1; j < n; ++j) {
            if (arr[i] == arr[j]) return arr[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;
}