How to toggle two fragments in one activity?

I have two fragment classes and two buttons in the application bar - each button is doing one of the following:

1. check if a fragment from the other classes exists
2. if exists, replace the container with new instance of his fragment class
3. if not exists create new instance of his fragment class

Here is the sample code:

private void openReader() {

    FragmentManager fragmentManager = getFragmentManager();

    ArticleListFragment fragmentList = (ArticleListFragment) fragmentManager.findFragmentById(R.id.action_list);
    ArticleReaderFragment fragmentReader = (ArticleReaderFragment) fragmentManager.findFragmentById(R.id.action_reader);

    if (fragmentReader==null || ! fragmentReader.isInLayout()) {

        if (fragmentList==null || ! fragmentList.isInLayout()) {

            FragmentTransaction ft = fragmentManager.beginTransaction();
            ft.add(R.id.container_id, new ArticleReaderFragment());
            //ft.addToBackStack(null);
            ft.commit();

        } else {

            FragmentTransaction ft = fragmentManager.beginTransaction();
            ft.replace(R.id.container_id, new ArticleReaderFragment());
            //ft.addToBackStack(null);
            ft.commit();

        }
    }
    else {
        // fragment.update(...);
    }

}

and here is the activity XML:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://ift.tt/nIICcg"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/welcome_page">

    <FrameLayout xmlns:android="http://ift.tt/nIICcg"
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/container_id">

    </FrameLayout>

</LinearLayout>

The issue is that the replace function is not destroying the old fragment class - it is adding more and more fragments over each other as the buttons are clicked.

Could anyone tell what I am doing wrong?