Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int i,j,c=1;
    for(i=0;i<n;i++){
        for(j=i+1;j<n;j++)
            if(arr[i] == arr[j])
                c++;
        if(c>1){
            //printf("%d\n",arr[i]);
            return arr[i];
            break;
        }
    }
    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