#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
const long long int N = 200005;
const long long int oo = 1000000000;
signed main() {
  long long int t;
  cin >> t;
  while (t--) {
    long long int n, m;
    cin >> n >> m;
    vector<long long int> a(n), K;
    set<long long int> s;
    map<long long int, bool> p;
    for (auto &i : a) {
      cin >> i;
      s.insert(i);
      if (!p[i]) {
        K.push_back(i);
        p[i] = 1;
      }
    }
    if (s.size() > m) {
      cout << "-1\n";
      continue;
    }
    long long int g = m - s.size();
    for (long long int i = 0; i < g; i++) {
      K.push_back(1);
    }
    cout << 100 * K.size() << '\n';
    for (long long int i = 0; i < 100; i++) {
      for (auto j : K) cout << j << ' ';
    }
    cout << '\n';
  }
  return 0;
}
