#include <stdio.h>

    void print_repeats(unsigned a[], unsigned n)
    {
        unsigned i, _2n = 2*n;
        for(i = 0; i < n; ++i) if(a[a[i] % n] < _2n) a[a[i] % n] += n;
        for(i = 0; i < n; ++i) if(a[i] >= _2n) printf("%u ", i);
        putchar('\n');
    }

    int main()
    {
        unsigned a[] = {1, 2, 3, 1, 3, 0, 3, 6};
        print_repeats(a, sizeof(a)/sizeof(unsigned));
        return 0;
    }