#include <bits/stdc++.h>
using namespace std;
long long int mx = INT_MAX;
long long int mn = INT_MIN;
long long int mod = 1e9 + 7;
const long long int maxs = 3e5 + 5;
long long int max2d = 1e3 + 3;
long long int INF = 1e18;
const long long int lg = 22;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  long long int T = 1;
  cin >> T;
  while (T--) {
    long long int N, K;
    cin >> N >> K;
    long long int A[N + 1];
    set<long long int> S;
    mx = INT_MIN;
    long long int flg[N + 1];
    memset(flg, 0, sizeof flg);
    for (long long int i = 1; i <= N; i++) {
      cin >> A[i];
      S.insert(A[i]);
      flg[A[i]] = 1;
      mx = max(mx, A[i]);
    }
    if ((long long int)S.size() > K) {
      cout << "-1"
           << "\n";
      continue;
    }
    cout << N * K << "\n";
    for (long long int i = 1; i <= N; i++) {
      long long int tmp = K - (long long int)S.size();
      for (auto it : S) {
        cout << it << " ";
      }
      while (tmp) {
        for (long long int j = 1; j <= N; j++) {
          if (flg[j] == 0) {
            cout << j << " ";
            tmp--;
            if (tmp == 0) break;
          }
        }
      }
    }
    cout << "\n";
  }
  return 0;
}
