#include <bits/stdc++.h>
using namespace std;
using ll = int;
using ull = unsigned long long;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vll = vector<ll>;
using vpll = vector<pll>;
using vvll = vector<vll>;
using vs = vector<string>;
struct TC {
  vll freq;
  int size;
  TC(int k) { freq.resize(k + 1); }
};
int32_t main(int argc, char** argv) noexcept {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int t = 1;
  cin >> t;
  while (t--) {
    int n, k;
    cin >> n >> k;
    vi v(n);
    set<int> s;
    for (int i = 0; i < n; ++i) {
      cin >> v[i];
      s.insert(v[i]);
    }
    if (s.size() > k) {
      cout << -1 << endl;
      continue;
    }
    vi ans;
    for (auto i : s) ans.emplace_back(i);
    int e = ans[ans.size() - 1];
    while (ans.size() != k) ans.emplace_back(e);
    cout << ans.size() * n << endl;
    for (int i = 0; i < n; ++i)
      for (auto j : ans) cout << j << ' ';
    cout << endl;
  }
  return 0;
}
