#include <bits/stdc++.h>
using namespace std;
int main() {
  int t;
  int n, k, a;
  cin >> t;
  while (t--) {
    cin >> n >> k;
    vector<int> v;
    set<int> s;
    for (int i = 0; i < n; i++) {
      cin >> a;
      v.push_back(a);
      s.insert(a);
    }
    if (n == k) {
      cout << n << endl;
      for (int i = 0; i < n; i++) {
        cout << v[i] << " ";
      }
      cout << endl;
      continue;
    } else if (s.size() > k) {
      cout << -1 << endl;
      continue;
    }
    for (int i = 1; i < n && s.size() < k; i++) {
      s.insert(i);
    }
    cout << n * s.size() << endl;
    for (int i = 0; i < n; i++) {
      for (set<int>::iterator itr = s.begin(); itr != s.end(); itr++) {
        cout << *itr << " ";
      }
    }
    cout << endl;
  }
}
