QCompleter makes parent QComboBox to call activated() signal twice

//main.cpp

#include <QApplication>
#include "form.h"


int main(int argc, char** argv)
{
    QApplication app(argc, argv);
    Form form;
    form.show();
    return app.exec();
}

// form.h

#ifndef FORM_H
#define FORM_H

#include <QWidget>

class Form : public QWidget
{
Q_OBJECT

public:
	Form();

private slots:
	void test_activated(int index);
};

#endif // FORM_H

// form.cpp

#include "form.h"

#include <QDebug>

#include <QComboBox>
#include <QCompleter>

Form::Form()
{
	resize(200, 50);
	QComboBox* p = new QComboBox(this);
	p->setEditable(true);
	p->setCompleter(new QCompleter(p));
	p->completer()->setCaseSensitivity(Qt::CaseInsensitive);

	p->setInsertPolicy(QComboBox::NoInsert);
	connect(p, SIGNAL(activated(int)), this, SLOT(test_activated(int)));

	p->completer()->setCompletionMode(QCompleter::PopupCompletion);
	p->completer()->setModel(p->model());

	QStringList items;
	items << "" << "Hello" << "Hair" << "World";
	p->addItems(items);
}

void Form::test_activated(int index)
{
	qDebug() << "Form::test_activated, index:" << index;
}

Form::test_activated, index: 1
Form::test_activated, index: 1