Why does activity continue to reboot after a power attack in android?

I have an Android app that integrates with Facebook. At the entrance to the system there is a code for integration into Facebook. After successfully logging into Facebook, my next snippet appears in MainActivity. The problem is that when the application closes MainActivity, it restarts itself again and again. Can anyone suggest a step by step solution for this? I submit my manifest code below:

<activity android:name="com.smacon.task.Login" android:label="@string/app_name">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>
<activity android:name="com.smacon.task.MainActivity" android:label="@string/app_name"
 android:launchMode="singleTask"/>