#include <bits/stdc++.h>
using namespace std;
signed main() {
  long long int t;
  cin >> t;
  while (t--) {
    long long int n, k;
    cin >> n >> k;
    long long int a[n];
    set<long long int> s;
    queue<long long int> q;
    for (long long int i = 0; i < n; i++) {
      cin >> a[i];
      if (s.find(a[i]) == s.end()) q.push(a[i]);
      s.insert(a[i]);
    }
    if (k < (long long int)s.size())
      cout << "-1\n";
    else {
      while ((long long int)q.size() != k) {
        q.push(1);
      }
      cout << n * k << "\n";
      for (long long int i = 0; i < n; i++) {
        queue<long long int> qq = q;
        while (!qq.empty()) {
          cout << qq.front() << " ";
          qq.pop();
        }
      }
      cout << "\n";
    }
  }
}
