#include <bits/stdc++.h>
using namespace std;
void INP();
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  if (0) return;
  cout << name << " : " << arg1 << '\n';
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  if (0) return;
  const char* comma = strchr(names + 1, ',');
  cout.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
void alpha() {
  long long int n, k;
  cin >> n >> k;
  long long int arr[n];
  unordered_map<long long int, long long int> mp;
  for (long long int i = 0; i < n; i++) {
    cin >> arr[i];
    mp[arr[i]]++;
  }
  if (mp.size() > k) {
    cout << -1 << '\n';
    return;
  }
  cout << n * k << '\n';
  long long int count = k - mp.size();
  for (long long int i = 0; i < n; i++) {
    for (auto i : mp) cout << i.first << " ";
    for (long long int i = 0; i < count; i++)
      cout << (*(mp.begin())).first << " ";
  }
  cout << '\n';
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int t = 1;
  cin >> t;
  while (t--) alpha();
  return 0;
}
inline void INP() {}
