#include <bits/stdc++.h>
using namespace std;
signed main(void) {
  long long int t = 1;
  cin >> t;
  while (t--) {
    long long int n, k;
    cin >> n >> k;
    long long int a[n];
    set<long long int> s;
    for (long long int i = 0; i < n; i++) {
      cin >> a[i];
      s.insert(a[i]);
    }
    if (k < s.size()) {
      cout << "-1" << endl;
    } else {
      cout << n * k << endl;
      for (long long int i = 1; i <= n; i++) {
        for (auto j : s) {
          cout << j << " ";
        }
        for (long long int i = 1; i <= (k - s.size()); i++) {
          cout << *s.rbegin() << " ";
        }
      }
      cout << endl;
    }
  }
}
