为什么我要回去回到我的主要活动？

Question

我在开发Android应用程序时遇到问题，当我尝试在我的片段视图中按下后退按钮时，它似乎接近同一片段的精确副本。如果我再按一次它然后进入我的主要活动（我想在第一次按下按钮时发生）？

这是我调用片段的窗口

    public class DaysWindow extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.activity_days_window);
        // Get the message from the intent
        Intent intent = getIntent();
        String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
        Scanner scan = new Scanner(message);
        scan.useDelimiter(";");

        if(savedInstanceState==null){



            DaysFragment days = new DaysFragment();
            days.setArguments(intent.getExtras());
            getSupportFragmentManager().beginTransaction()
            .add(R.id.container, days).commit();

        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.days_window, menu);
    return true;
}

    }

这是我看过两次的片段

    public class DaysFragment extends android.support.v4.app.Fragment {
String days;
String message;


public View onCreateView(LayoutInflater inflater, ViewGroup Container, Bundle       SavedInstanceState){

    View view=inflater.inflate(R.layout.fragment_days, Container, false);
    days=getArguments().getString(MainActivity.EXTRA_MESSAGE);
    TextView textView= (TextView) view.findViewById(R.id.DaysTextView);
    Scanner scan = new Scanner(days);
    scan.useDelimiter(";");
    textView.setText(scan.next());


    return view;
    }
    }