My Desired requirement was NavigationDrawer-->Fragment1-->Fragment2--> On BackButtonPress-->Fragment1(On BackButton Pressed)-->MainActivity.

But,Now i am only able to move to Fragment1,But cant move to Fragment2. It shows an error java.lang.IllegalStateException: commit already called

In My MainActivity

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    FragmentManager fragmentManager = getSupportFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    // Handle navigation view item clicks here.
    int id = item.getItemId();
    RelativeLayout mainLayout=(RelativeLayout)findViewById(R.id.mainlayout);

  if (id == R.id.nav_project) {
        ProjectFragment fragment = new ProjectFragment();
        mainLayout.removeAllViews();
        fragmentTransaction.add(R.id.mainlayout, fragment);
        fragmentTransaction.addToBackStack(null).commit();

    } 

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}
 @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }

My First Fragment

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.RelativeLayout;


/**
 * A simple {@link Fragment} subclass.
 */
public class LoginFragment extends Fragment {
    Button signin,login;
    ImageView logo;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_login,container,false);
        FragmentManager fragmentManager = getFragmentManager();
        final FragmentTransaction fragmentTransaction =
                fragmentManager.beginTransaction();
        final RelativeLayout mainLayout=(RelativeLayout)getActivity().findViewById(R.id.mainlayout);
        signin=(Button)v.findViewById(R.id.button2);

        signin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                SignUpFragment fragment = new SignUpFragment();
                mainLayout.removeAllViews();
                fragmentTransaction.add(R.id.mainlayout, fragment);
                fragmentTransaction.commit();
                fragmentTransaction.addToBackStack(null).commit();
            }
        });
        login=(Button)v.findViewById(R.id.button);
        login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                LoginEnterFragment fragment = new LoginEnterFragment();
                mainLayout.removeAllViews();
                fragmentTransaction.add(R.id.mainlayout, fragment);
                fragmentTransaction.commit();
                fragmentTransaction.addToBackStack(null).commit();
            }
        });
        logo=(ImageView)v.findViewById(R.id.imageView);

        return v;
    }

}

My third Fragment

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;



/**
 * A simple {@link Fragment} subclass.
 */
public class LoginEnterFragment extends Fragment {

    public LoginEnterFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_login_enter, container, false);
    }


}

My Logcat

11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime: FATAL EXCEPTION: main
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime: Process: com.example.rubin, PID: 676
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime: java.lang.IllegalStateException: commit already called
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:621)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:613)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at com.example.root5solutions.mirrealtors.LoginFragment$2.onClick(LoginFragment.java:53)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at android.view.View.performClick(View.java:4780)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at android.view.View$PerformClick.run(View.java:19866)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at android.os.Handler.handleCallback(Handler.java:739)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:95)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:135)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5254)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:372)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
11-01 23:44:17.237 676-676/com.example.rubin E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

IllegalStateException: commit already called

Because calling commit() method two time in LoginFragment Fragment on signin and login Button click :

fragmentTransaction.commit();
fragmentTransaction.addToBackStack(null).commit();

lines causing issue.

Use addToBackStack method as:

fragmentTransaction.addToBackStack(null).commit();