#include <bits/stdc++.h>
using namespace std;
const long long int MOD = 1e9 + 7;
void solve() {
  long long int n, a, k;
  cin >> n >> k;
  set<long long int> st;
  vector<long long int> v;
  for (long long int i = 0; i < n; i++) {
    cin >> a;
    v.push_back(a);
    st.insert(a);
  }
  if (st.size() > k) {
    cout << "-1" << endl;
    return;
  }
  vector<long long int> rp;
  for (auto it = st.begin(); it != st.end(); it++) {
    rp.push_back(*it);
  }
  while (rp.size() < k) {
    rp.push_back(rp[0]);
  }
  long long int cnt = 0;
  cout << n * rp.size() << endl;
  while (cnt < n) {
    for (long long int i = 0; i < rp.size(); i++) {
      cout << rp[i] << " ";
    }
    cnt += 1;
  }
  cout << endl;
}
int main(void) {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int t;
  cin >> t;
  while (t--) solve();
  return 0;
}
