Android で「指定された子にはすでに親がいます」というエラーが表示されるのはなぜですか?

FATAL EXCEPTION: main
java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.

<code class="java">tv = new TextView(getApplicationContext()); // initialized elsewhere
et = new EditText(getApplicationContext()); // initialized elsewhere

private void ConsoleWindow() {
    runOnUiThread(new Runnable() {
        @Override
        public void run() {
            // MY LAYOUT:
            setContentView(R.layout.activity_console);
            // LINEAR LAYOUT
            LinearLayout layout = new LinearLayout(getApplicationContext());
            layout.setOrientation(LinearLayout.VERTICAL);
            setContentView(layout);

            // TEXTVIEW
            layout.addView(tv); // ERROR IN THIS LINE DURING 2ND RUN
            // EDITTEXT
            et.setHint("Enter Command");
            layout.addView(et);
        }
    });
}</code>

<code class="java">if (tv.getParent() != null) {
    ((ViewGroup) tv.getParent()).removeView(tv); // Fix
}</code>