#include <bits/stdc++.h>
using namespace std;
const long long m = 1e9 + 7;
long long power(long long x, long long y) {
  long long res = 1;
  x = x;
  while (y > 0) {
    if (y & 1) res = (res * x);
    y = y >> 1;
    x = (x * x);
  }
  return res;
}
long long logtwo(long long n) {
  if (n == 1) return 0;
  return logtwo(n / 2) + 1;
}
long long binV(vector<long long> arr, long long x) {
  long long l = 0;
  long long r = arr.size() - 1;
  while (l <= r) {
    long long m = l + (r - l) / 2;
    if (arr[m] == x) return m;
    if (arr[m] > x)
      r = m - 1;
    else
      l = m + 1;
  }
  return -1;
}
long long binA(long long arr[], long long l, long long r, long long x) {
  while (l <= r) {
    long long m = l + (r - l) / 2;
    if (arr[m] == x) return m;
    if (arr[m] > x)
      r = m - 1;
    else
      l = m + 1;
  }
  return -1;
}
vector<long long> primeV;
bool isPrime(long long x) {
  for (long long i = 0; i < primeV.size(); i++)
    if (x % primeV[i] == 0) return false;
  return true;
}
void makePrime() {
  primeV.push_back(2);
  for (long long i = 3; i <= 1001; i += 2) {
    if (isPrime(i)) primeV.push_back(i);
  }
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t;
  cin >> t;
  while (t--) {
    long long n, k;
    cin >> n >> k;
    long long arr[n];
    vector<long long> v;
    map<long long, long long> mp;
    for (long long i = 0; i < n; i++) {
      cin >> arr[i];
      mp[arr[i]]++;
      if (mp[arr[i]] == 1) v.push_back(arr[i]);
    }
    if (v.size() > k) {
      cout << -1 << "\n";
      continue;
    }
    for (long long i = v.size(); i < k; i++) v.push_back(arr[i]);
    cout << v.size() * n << "\n";
    for (long long i = 0; i < n * v.size(); i++) {
      cout << v[i % v.size()] << " ";
    }
    cout << "\n";
  }
}
