#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t;
  cin >> t;
  while (t > 0) {
    --t;
    long long n, k;
    cin >> n >> k;
    vector<long long> a(n);
    set<long long> sad;
    vector<long long> ord;
    long long mx = 0;
    for (long long i = 0; i < n; ++i) {
      cin >> a[i];
      long long l = sad.size();
      sad.insert(a[i]);
      if (l != sad.size()) {
        ord.push_back(a[i]);
      }
    }
    if (sad.size() > k) {
      cout << -1 << endl;
    } else {
      cout << n * k << endl;
      for (long long i = 0; i < n; ++i) {
        for (long long j = 0; j < k; ++j) {
          if (j < ord.size()) {
            cout << ord[j] << " ";
          } else {
            cout << 1 << " ";
          }
        }
      }
      cout << endl;
    }
  }
}
