int input[N];

int i;
for (i = 0; i < N; i++) {
    int num = input[i];
    if (num < 0) {
        num = -num;
    }
    input[num] = -input[num];
    if (input[num] >= 0) {
        printf("%d is repeated\n", num);
        break;
    }
}