#include <bits/stdc++.h>
using namespace std;
const int N = 2e6 + 5;
int t, n, k;
long long arr[N];
int main() {
  cin >> t;
  while (t--) {
    cin >> n >> k;
    set<long long> st;
    vector<long long> val;
    for (int i = 0; i < n; i++) {
      cin >> arr[i];
      st.insert(arr[i]);
    }
    if (st.size() > k) {
      cout << -1 << endl;
      continue;
    }
    for (long long x : st) {
      val.push_back(x);
    }
    vector<long long> ans;
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < k; j++) {
        ans.push_back(val[j % (val.size())]);
      }
    }
    cout << ans.size() << endl;
    for (long long x : ans) {
      cout << x << " ";
    }
    cout << endl;
  }
  return 0;
}
