Крашится приложение при замене фрагмента

Когда я нажимаю на кнопку замены фрагмента, приложение вылетает.

MainActivity.java:

public class MainActivity extends AppCompatActivity {

Button btn1, btn2;
FragmentManager fragmentManager;
FragmentTransaction fragmentTransaction;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    btn1 = findViewById(R.id.button);
    btn2 = findViewById(R.id.button2);
    fragmentManager = getSupportFragmentManager();
    fragmentTransaction = 
    fragmentManager.beginTransaction();
    final BlankFragment blankFragment = new 
    BlankFragment();

    btn1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(fragmentTransaction.isEmpty()){
                fragmentTransaction.add(R.id.fragment, 
    blankFragment).commit();
            } else{
                fragmentTransaction.replace(R.id.fragment, 
    blankFragment).commit();
            }
        }
    });

    btn2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(fragmentTransaction.isEmpty()){
                fragmentTransaction.add(R.id.fragment, new 
    BlankFragment2()).commit();}
            else{
            fragmentTransaction.replace(R.id.fragment, new 
    BlankFragment2()).commit();}
        }
        });
      }
    }

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">

<Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Button"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintStart_toStartOf="parent" />

<Button
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Button"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent" />

<fragment
    android:id="@+id/fragment"
    android:name="com.example.test.view.BlankFragment"
    android:layout_width="0dp"
    android:layout_height="0dp"
    app:layout_constraintBottom_toTopOf="@+id/button"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

BlankFragment.java

public class BlankFragment extends Fragment {

public BlankFragment() {}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup 
container, Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_blank, 
container, false);
   }
}

fragment_blank.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".view.BlankFragment"
android:background="@color/colorAccent">
</FrameLayout>

BlankFragment2.java

public class BlankFragment2 extends Fragment {

public BlankFragment2() {}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup 
container, Bundle savedInstanceState) {
    return 
    inflater.inflate(R.layout.fragment_blank_fragment2, 
    container, false);
    }
}

fragment_blank_fragment2.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".view.BlankFragment2"
    android:background="@color/colorPrimaryDark"/>

Logcat

04-05 16:33:56.059 3504-3504/com.example.test E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.test, PID: 3504
    java.lang.IllegalStateException: Fragment already added: BlankFragment2{163c85c5 #1 id=0x7f070042}
        at androidx.fragment.app.FragmentManagerImpl.addFragment(FragmentManager.java:1916)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:765)
        at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManager.java:2625)
        at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411)
        at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2366)
        at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2273)
        at androidx.fragment.app.FragmentManagerImpl$1.run(FragmentManager.java:733)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at