Android – intent extras are duplicated when using FLAG_UPDATE_CURRENT in PendingIntent when creating android notifications

The way I solved that problem was by assigning a unique requestCode when you get the PendingIntent:

PendingIntent.getActivity(context, requestCode, showIntent, 0); 

By doing so you are registering with the system different/unique intent instances. Tip: A good way of making the requestCode unique would be by passing to it the current system time.

int requestID = (int) System.currentTimeMillis();

(Original answer revised, see edit history for it)

I'm not actually sure, which part of your system is malfunctioning so I post my test code here, which I tested to work correctly. First there is MyService, then TestActivity which displays the car details in DetailsActivity:

CarService.java

public class CarService extends IntentService
{
    public CarService()
    {
        super("CarService");
    }

    protected void onHandleIntent(Intent intent)
    {
        Bundle extras = intent.getExtras();
        if (extras == null)
        {
            Log.e("CarService", "Service onHandleIntent null extras");
            return;
        }

        int carId = extras.getInt("carId");
        String carName = extras.getString("name");
        Log.i("CarService", "Service onHandleIntent car = " + carName + " with ID = " + Integer.toString(carId));

        Intent notificationIntent;
        notificationIntent = new Intent(this, DetailsActivity.class);
        notificationIntent.putExtra("carId", carId);
        notificationIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

        PendingIntent pending = PendingIntent.getActivity(this, carId, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT);

        Notification notif = new Notification(R.drawable.icon, carName, System.currentTimeMillis());
        notif.flags |= Notification.FLAG_AUTO_CANCEL;
        notif.setLatestEventInfo(getApplicationContext(), carName, "Car Details", pending);

        NotificationManager mNotificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        mNotificationManager.notify(carId, notif);
    }
}

TestActivity.java (your main activity)

public class TestActivity extends Activity implements OnClickListener
{
    @Override public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.test);

        Button btn = (Button) findViewById(R.id.launch);
        btn.setOnClickListener(this);
    }

    @Override public void onClick(View v)
    {
        startActivity(new Intent(this, DetailsActivity.class));
    }
}

test.xml (layout for TestActivity.java)

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:a="http://schemas.android.com/apk/res/android" a:id="@+id/layout_root" a:orientation="vertical" a:layout_width="fill_parent" a:layout_height="fill_parent">
    <TextView a:id="@+id/test_value"  a:text="Main..." a:layout_width="wrap_content" a:layout_height="wrap_content"/>
    <Button a:id="@+id/launch" a:text="Details" a:layout_width="100dp" a:layout_height="wrap_content"/>
</LinearLayout>

DetailsActivity.java (car details listed here + launches CarService + notifications lead back to here)

public class DetailsActivity extends Activity implements OnClickListener
{
    private String[] cars = new String[]{"Ferrari", "Lamborghini", "Lada", "Nissan", "Opel", "Bugatti"};
    private int id = 0;


    @Override public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.details);

        Button btn = (Button) findViewById(R.id.doit);
        btn.setOnClickListener(this);

        Bundle extras = getIntent().getExtras();
        if (extras != null)
        {
            final int id = extras.getInt("carId");
            Log.i("DetailsActivity", "Car ID: " + id);
            TextView tv = (TextView) findViewById(R.id.test_value);
            tv.setText("Car ID = " + Integer.toString(id) + ", which is " + cars[id%6]); // id%6 prevents a crash with the string array when clicking test button over 6 times
        }
    }

    @Override public void onClick(View v)
    {
        Intent intent = new Intent(this, CarService.class);
        intent.putExtra("carId", id);
        intent.putExtra("name", cars[id%6]); // id%6 prevents a crash with the string array when clicking test button over 6 times
        startService(intent);
        ++id;
    }
}

details.xml (layout for TestActivity.java)

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:a="http://schemas.android.com/apk/res/android" a:orientation="vertical" a:layout_width="fill_parent" a:layout_height="fill_parent">
    <TextView a:id="@+id/test_value" a:text="No details yet: click the button." a:layout_width="wrap_content" a:layout_height="wrap_content"/>
    <Button a:id="@+id/doit" a:text="Test" a:layout_width="100dp" a:layout_height="wrap_content"/>
</LinearLayout>

I hope all that works correctly as I did make some minor changes on the fly.