#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t;
  cin >> t;
  for (int test = 0; test < t; test++) {
    int n, k;
    cin >> n >> k;
    vector<int> v(n);
    set<int> nums;
    for (int i = 0; i < n; i++) {
      cin >> v[i];
      nums.insert(v[i]);
    }
    if (nums.size() > k) {
      cout << -1 << endl;
      continue;
    }
    for (int i = 1; i <= k; i++) {
      if (nums.size() == k) break;
      nums.insert(i);
    }
    vector<int> construct;
    for (int elem : nums) {
      construct.emplace_back(elem);
    }
    cout << 10000 << endl;
    for (int i = 0; i < 10000; i++) {
      if (i != 0) cout << " ";
      cout << construct[i % k];
    }
    cout << endl;
  }
  return 0;
}
