Find Duplicate in Range 0 and n-1

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int seen[100] = {0};  // frequency array up to size 100

    for (int i = 0; i < n; i++) {
        if (seen[arr[i]]) {
            return arr[i];  // first duplicate found
        }
        seen[arr[i]] = 1;
    }

    return -1;  // no duplicate found
}

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;
}