#include <bits/stdc++.h>
using namespace std;
const int N = 1e5;
void solve() {
  int n, k;
  cin >> n >> k;
  vector<int> a(n);
  for (int i = 0; i < n; i++) cin >> a[i];
  set<int> distinct;
  for (int i = 0; i < n; i++) distinct.insert(a[i]);
  if (distinct.size() > k) {
    cout << -1 << "\n";
    return;
  }
  {
    vector<int> ans;
    for (int i = 0; i < n; i++) {
      for (int j : distinct) {
        ans.push_back(j);
      }
      for (int j = distinct.size(); j < k; j++) ans.push_back(1);
    }
    cout << ans.size() << "\n";
    for (int i : ans) cout << i << " ";
    cout << "\n";
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie();
  cout.tie();
  int t = 1;
  cin >> t;
  while (t--) {
    solve();
  }
  return 0;
}
