#include <bits/stdc++.h>
using namespace std;
void solve_case() {
  int N, K;
  cin >> N >> K;
  vector<int> a(N);
  vector<int> uniques;
  vector<bool> seen(N + 1, false);
  for (int i = 0; i < N; ++i) {
    cin >> a[i];
    if (!seen[a[i]]) {
      uniques.push_back(a[i]);
      seen[a[i]] = true;
    }
  }
  if (uniques.size() > K) {
    cout << "-1\n";
    return;
  }
  while (uniques.size() < K) {
    uniques.push_back(1);
  }
  cout << N * uniques.size() << "\n";
  for (int i = 0; i < N; ++i) {
    for (int j = 0; j < uniques.size(); ++j) {
      cout << uniques[j];
      if (i == N - 1 && j == uniques.size() - 1) {
        cout << "\n";
      } else {
        cout << " ";
      }
    }
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int T;
  cin >> T;
  for (int t = 0; t < T; ++t) {
    solve_case();
  }
  return 0;
}
