#include <bits/stdc++.h>
using namespace std;
signed main() {
  long long int t;
  cin >> t;
  while (t--) {
    long long int n, k;
    cin >> n >> k;
    vector<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]);
    }
    bool ans = true;
    if (s.size() > k) {
      ans = false;
    }
    if (ans == false)
      cout << -1 << "\n";
    else {
      cout << n * k << "\n";
      for (long long int i = 0; i < n; i++) {
        for (auto a : s) {
          cout << a << " ";
        }
        for (long long int j = 0; j < k - s.size(); j++) {
          cout << 1 << " ";
        }
      }
      cout << "\n";
    }
  }
  return 0;
}
