#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int i, n, t, j = 0, p, x, l = 0, r = 0, y, k, c, sum = 0, m, d,
                         cnts = 0;
  cin >> t;
  while (t--) {
    cin >> n >> k;
    set<int> se;
    for (i = 0; i < n; i++) {
      cin >> c;
      se.insert(c);
    }
    if (se.size() > k) {
      cout << -1 << endl;
    } else {
      int ara[n];
      cout << n * k << endl;
      j = 0;
      for (auto it : se) {
        ara[j++] = it;
      }
      p = ara[j - 1];
      for (i = j; i < k; i++) {
        ara[i] = p;
      }
      p = n;
      x = 0;
      while (x < p) {
        for (i = 0; i < k; i++) {
          cout << ara[i] << " ";
        }
        x++;
      }
      cout << endl;
    }
  }
}
