#include <bits/stdc++.h>
using namespace std;
long long a[100500], b[100050];
set<long long> st;
int main() {
  long long i, j, k, l, m, n, t, d, x, zr = 0;
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin >> t;
  while (t--) {
    cin >> n >> k;
    for (i = 0; i < n; i++) {
      cin >> a[i];
      st.insert(a[i]);
    }
    l = st.size();
    vector<long long> v;
    if (l <= k) {
      cout << n * k << endl;
      for (auto it = st.begin(); v.size() < k;) {
        v.push_back(*it);
        it++;
        if (it == st.end()) {
          it = st.begin();
        }
      }
      for (i = 0; i < n; i++) {
        for (j = 0; j < k; j++) {
          cout << v[j] << " ";
        }
      }
      cout << endl;
    } else {
      cout << -1 << endl;
    }
    st.clear();
  }
  return 0;
}
