#include <bits/stdc++.h>
using namespace std;
int main() {
  int t;
  cin >> t;
  while (t--) {
    int n, m;
    cin >> n >> m;
    int a[n];
    set<int> s;
    for (int i = 0; i < n; i++) {
      cin >> a[i];
      s.insert(a[i]);
    }
    if (s.size() > m) {
      cout << -1 << endl;
      continue;
    }
    int p = 1;
    while (s.size() < m) {
      s.insert(p);
      p++;
    }
    int tk = 0;
    vector<int> ans;
    while (tk < m * n) {
      for (auto x : s) {
        ans.push_back(x);
      }
      tk += (int)s.size();
    }
    cout << ans.size() << endl;
    for (auto x : ans) {
      cout << x << " ";
    }
    cout << endl;
  }
}
