#include <bits/stdc++.h>
using namespace std;
int main() {
  int t;
  cin >> t;
  while (t--) {
    int n, k;
    cin >> n >> k;
    int a[n];
    int top = -1;
    unordered_map<int, int> vis;
    for (int i = 0; i < n; i++) {
      int x;
      cin >> x;
      if (!vis[x]) a[++top] = x;
      vis[x] = 1;
    }
    if (top > k - 1)
      cout << "-1\n";
    else {
      while (top + 1 < k) a[++top] = 1;
      cout << n * k << "\n";
      for (int i = 0; i < n; i++) {
        for (int j = 0; j < k; j++) cout << a[j] << " ";
      }
      cout << "\n";
    }
  }
  return 0;
}
