Starting Activity through notification: Avoiding duplicate activities

Question

So I am currently showing a notification. When the user clicks this noticiation, the application is started. The notification persists, to indicate that the service is runni

Answer 1

As the two answers above have mentioned, you'll want to set the application's launch mode which is defined in the activity's definition in the manifest:

<activity
    android:name="com.company.ActivityName"
    android:launchMode="singleTask">
</activity>

Additionally, you may want to note, that despite FLAG_ACTIVITY_SINGLE_TOP being a valid Intent flag, there are no equivalent intent flags for singleTask or singleInstance.

See the launchMode section for more details on the different launch mode options: http://developer.android.com/guide/topics/manifest/activity-element.html#lmode

Answer 2

That's the way it's supposed to be by default. You probably need to specify android:launchMode="singleTop" if you want to have a single instance only.
There are 4 launch modes, more info here: https://developer.android.com/guide/topics/manifest/activity-element.html

Answer 3

When using the lanchMode="singleTask", if an instance of your activity already exists, Android does not re-create the Activity but launch it with the onNewIntent() method.

As documented by Android :

The system creates the activity at the root of a new task and routes the intent to it. However, if an instance of the activity already exists, the system routes the intent to existing instance through a call to its onNewIntent() method, rather than creating a new one.

Android documentation for activity mode