#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long t;
  t = 1;
  cin >> t;
  while (t--) {
    long long n, m, i;
    cin >> n >> m;
    long long arr[n];
    vector<long long> vec;
    set<long long> st;
    for (i = 0; i < n; i++) {
      cin >> arr[i];
      st.insert(arr[i]);
    }
    for (auto x : st) vec.push_back(x);
    if (st.size() > m) {
      cout << -1 << '\n';
      continue;
    }
    if (n == m) {
      cout << n << '\n';
      for (auto x : arr) cout << x << " ";
      cout << '\n';
    } else {
      long long left = m - vec.size();
      while (left--) vec.push_back(1);
      cout << n * m << '\n';
      while (n--) {
        for (auto x : vec) cout << x << " ";
      }
      cout << '\n';
    }
  }
  return 0;
}
