#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using dd = double;
using pll = pair<ll, ll>;
using tll = tuple<ll, ll, ll>;
using vll = vector<ll>;
using vdd = vector<dd>;
using vpll = vector<pll>;
using vtll = vector<tll>;
using vvll = vector<vll>;
using vvdd = vector<vdd>;
using vvpll = vector<vpll>;
using vvtll = vector<vtll>;
using vvvll = vector<vvll>;
using vvvdd = vector<vvdd>;
using vvvpll = vector<vvpll>;
using vvvtll = vector<vvtll>;
using vvvvll = vector<vvvll>;
using vvvvdd = vector<vvvdd>;
using vvvvpll = vector<vvvpll>;
using vvvvtll = vector<vvvtll>;
constexpr ll INF = 1LL << 60;
constexpr dd EPS = 1e-11;
struct Fast {
  Fast() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
  }
} fast;
template <class T>
inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline T MaxE(vector<T> &v, ll S, ll E) {
  T m = v[S];
  for (ll i = (S); i <= (E); i++) chmax(m, v[i]);
  return m;
}
template <class T>
inline T MinE(vector<T> &v, ll S, ll E) {
  T m = v[S];
  for (ll i = (S); i <= (E); i++) chmin(m, v[i]);
  return m;
}
template <class T>
inline T MaxE(vector<T> &v, ll N) {
  return MaxE(v, 0, N - 1);
}
template <class T>
inline T MinE(vector<T> &v, ll N) {
  return MinE(v, 0, N - 1);
}
template <class T>
inline T MaxE(vector<T> &v) {
  return MaxE(v, (ll)v.size());
}
template <class T>
inline T MinE(vector<T> &v) {
  return MinE(v, (ll)v.size());
}
template <class T>
inline ll MaxI(vector<T> &v, ll S, ll E) {
  ll m = S;
  for (ll i = (S); i <= (E); i++) {
    if (v[i] > v[m]) m = i;
  }
  return m;
}
template <class T>
inline ll MinI(vector<T> &v, ll S, ll E) {
  ll m = S;
  for (ll i = (S); i <= (E); i++) {
    if (v[i] < v[m]) m = i;
  }
  return m;
}
template <class T>
inline ll MaxI(vector<T> &v, ll N) {
  return MaxI(v, 0, N - 1);
}
template <class T>
inline ll MinI(vector<T> &v, ll N) {
  return MinI(v, 0, N - 1);
}
template <class T>
inline ll MaxI(vector<T> &v) {
  return MaxI(v, (ll)v.size());
}
template <class T>
inline ll MinI(vector<T> &v) {
  return MinI(v, (ll)v.size());
}
template <class T>
inline T Sum(vector<T> &v, ll S, ll E) {
  T s = v[S];
  for (ll i = (S + 1); i <= (E); i++) s += v[i];
  return s;
}
template <class T>
inline T Sum(vector<T> &v, ll N) {
  return Sum(v, 0, N - 1);
}
template <class T>
inline T Sum(vector<T> &v) {
  return Sum(v, v.size());
}
template <class T>
inline ll sz(T &v) {
  return (ll)v.size();
}
template <class T>
inline T POW(T a, ll n) {
  T r = 1;
  for (; n > 0; n >>= 1, a *= a) {
    if (n & 1) r *= a;
  }
  return r;
}
inline ll POW(int a, ll n) { return POW((ll)a, n); }
inline ll CEIL(ll a, ll b) { return (a + b - 1) / b; }
ll Gcd(ll a, ll b) { return (b == 0) ? a : Gcd(b, a % b); }
ll Lcm(ll a, ll b) { return a * b / Gcd(a, b); }
inline ll MSB(ll a) {
  for (ll o = 63, x = -1;;) {
    ll m = (o + x) / 2;
    if (a < (1LL << m))
      o = m;
    else
      x = m;
    if (o - x == 1) return x;
  }
}
inline ll Upper2N(ll s) { return 1LL << MSB(s) << 1; }
inline vll Bit2Idx(ll m) {
  vll v;
  for (ll i = 0; m; m >>= 1, i++)
    if (m & 1) v.push_back(i);
  return move(v);
}
inline ll BitNum(ll m) {
  for (ll c = 0;; m >>= 1) {
    c += m & 1;
    if (!m) return c;
  }
}
inline ll Bit(ll s, ll i) { return (s >> i) & 1; }
[[nodiscard]] inline ll BitOn(ll s, ll i) { return s | (1LL << i); }
[[nodiscard]] inline ll BitOff(ll s, ll i) { return s & ~(1LL << i); }
[[nodiscard]] inline ll BitCut(ll s, ll i) { return s & (1LL << ++i) - 1; }
inline vll str2num(string &s, ll N) {
  vll v(N);
  for (ll i = (0); i <= ((N)-1); i++) v[i] = (ll)(s[i] - '0');
  return move(v);
}
inline vll ALP2num(string &s, ll N) {
  vll v(N);
  for (ll i = (0); i <= ((N)-1); i++) v[i] = (ll)(s[i] - 'A');
  return move(v);
}
inline vll alp2num(string &s, ll N) {
  vll v(N);
  for (ll i = (0); i <= ((N)-1); i++) v[i] = (ll)(s[i] - 'a');
  return move(v);
}
template <class T>
inline ll UnderNumOf(vector<T> &v, T x) {
  return upper_bound(v.begin(), v.end(), x) - v.begin();
}
template <class T, class Pr>
inline ll UnderNumOf(vector<T> &v, T x, Pr pred) {
  return upper_bound(v.begin(), v.end(), x, pred) - v.begin();
}
template <class T>
inline ll OverNumOf(vector<T> &v, T x) {
  return (ll)v.size() - (lower_bound(v.begin(), v.end(), x) - v.begin());
}
template <class T, class Pr>
inline ll OverNumOf(vector<T> &v, T x, Pr pred) {
  return (ll)v.size() - (lower_bound(v.begin(), v.end(), x, pred) - v.begin());
}
template <class T = ll>
inline vector<T> cinv(ll N) {
  vector<T> v(N);
  for (ll i = (0); i <= ((N)-1); i++) cin >> v[i];
  return move(v);
}
template <class T = ll, class S = ll>
inline vector<pair<T, S>> cinv2(ll N) {
  vector<pair<T, S>> v(N);
  for (ll i = (0); i <= ((N)-1); i++) {
    cin >> v[i].first >> v[i].second;
  }
  return move(v);
}
template <class T = ll, class S = ll, class R = ll>
inline vector<tuple<T, S, R>> cinv3(ll N) {
  vector<tuple<T, S, R>> v(N);
  for (ll i = (0); i <= ((N)-1); i++) {
    cin >> get<0>(v[i]) >> get<1>(v[i]) >> get<2>(v[i]);
  }
  return move(v);
}
template <class T = ll, class S = ll, class R = ll, class Q = ll>
inline vector<tuple<T, S, R, Q>> cinv4(ll N) {
  vector<tuple<T, S, R, Q>> v(N);
  for (ll i = (0); i <= ((N)-1); i++) {
    cin >> get<0>(v[i]) >> get<1>(v[i]) >> get<2>(v[i]) >> get<3>(v[i]);
  }
  return move(v);
}
template <class T = ll>
inline vector<vector<T>> cinvv(ll N, ll M) {
  vector<vector<T>> vv(N);
  for (ll i = (0); i <= ((N)-1); i++) vv[i] = cinv(M);
  return move(vv);
}
template <class T>
inline void coutv(vector<T> &v, char deli = ' ') {
  ll N = (ll)v.size();
  for (ll i = (0); i <= ((N)-1); i++) {
    cout << v[i] << ((i == N - 1) ? '\n' : deli);
  }
}
template <class T>
void bye(T a) {
  cout << a << '\n';
  exit(0);
}
template <class T>
void Dump(vector<T> &v) {}
template <class T>
void Dump(vector<vector<T>> &v) {}
template <class T>
void Dump(vector<vector<vector<T>>> &v) {}
template <class T>
void Dump(vector<vector<vector<vector<T>>>> &v) {}
template <class T>
void Dump(vector<vector<vector<vector<vector<T>>>>> &v) {}
template <ll MOD>
struct mll_ {
  ll val;
  mll_(ll v = 0) : val(v % MOD) {
    if (val < 0) val += MOD;
  }
  mll_ operator-() const { return -val; }
  mll_ operator+(const mll_ &b) const { return val + b.val; }
  mll_ operator-(const mll_ &b) const { return val - b.val; }
  mll_ operator*(const mll_ &b) const { return val * b.val; }
  mll_ operator/(const mll_ &b) const { return mll_(*this) /= b; }
  mll_ operator+(ll b) const { return *this + mll_(b); }
  mll_ operator-(ll b) const { return *this - mll_(b); }
  mll_ operator*(ll b) const { return *this * mll_(b); }
  friend mll_ operator+(ll a, const mll_ &b) { return b + a; }
  friend mll_ operator-(ll a, const mll_ &b) { return -b + a; }
  friend mll_ operator*(ll a, const mll_ &b) { return b * a; }
  friend mll_ operator/(ll a, const mll_ &b) { return mll_(a) / b; }
  mll_ &operator+=(const mll_ &b) {
    val = (val + b.val) % MOD;
    return *this;
  }
  mll_ &operator-=(const mll_ &b) {
    val = (val + MOD - b.val) % MOD;
    return *this;
  }
  mll_ &operator*=(const mll_ &b) {
    val = (val * b.val) % MOD;
    return *this;
  }
  mll_ &operator/=(const mll_ &b) {
    ll c = b.val, d = MOD, u = 1, v = 0;
    while (d) {
      ll t = c / d;
      c -= t * d;
      swap(c, d);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0) val += MOD;
    return *this;
  }
  mll_ &operator+=(ll b) { return *this += mll_(b); }
  mll_ &operator-=(ll b) { return *this -= mll_(b); }
  mll_ &operator*=(ll b) { return *this *= mll_(b); }
  mll_ &operator/=(ll b) { return *this /= mll_(b); }
  bool operator==(const mll_ &b) { return val == b.val; }
  bool operator!=(const mll_ &b) { return val != b.val; }
  bool operator==(ll b) { return *this == mll_(b); }
  bool operator!=(ll b) { return *this != mll_(b); }
  friend bool operator==(ll a, const mll_ &b) { return mll_(a) == b.val; }
  friend bool operator!=(ll a, const mll_ &b) { return mll_(a) != b.val; }
  friend ostream &operator<<(ostream &os, const mll_ &a) { return os << a.val; }
  friend istream &operator>>(istream &is, mll_ &a) { return is >> a.val; }
  static mll_ Combination(ll a, ll b) {
    chmin(b, a - b);
    if (b < 0) return mll_(0);
    mll_ c = 1;
    for (ll i = (0); i <= ((b)-1); i++) c *= a - i;
    for (ll i = (0); i <= ((b)-1); i++) c /= i + 1;
    return c;
  }
};
using mll = mll_<1000000007LL>;
using vmll = std::vector<mll>;
using vvmll = std::vector<vmll>;
using vvvmll = std::vector<vvmll>;
using vvvvmll = std::vector<vvvmll>;
using vvvvvmll = std::vector<vvvvmll>;
void solve() {
  ll N, K;
  cin >> N >> K;
  vector<ll> A = cinv<ll>(N);
  vll cnt(N + 1);
  for (ll i = (0); i <= (N - 1); i++) {
    if (cnt[A[i]] == 0) cnt[A[i]]++;
  }
  ll KK = Sum(cnt);
  if (KK > K) {
    cout << -1 << '\n';
    return;
  }
  vll base;
  for (ll i = (1); i <= (N); i++) {
    if (cnt[i] > 0) {
      base.push_back(i);
    }
  }
  ll NN = sz(base);
  for (ll i = (NN); i <= (K - 1); i++) {
    base.push_back(base[i % NN]);
  }
  vll ans;
  ll idx = 0;
  ll j = 0;
  while (true) {
    if (idx >= N) break;
    ll a = base[j];
    ans.push_back(a);
    if (a == A[idx]) idx++;
    j++;
    j %= K;
  }
  cout << sz(ans) << '\n';
  coutv(ans);
}
int main() {
  ll t;
  cin >> t;
  for (ll i = (0); i <= (t - 1); i++) solve();
  return 0;
}
