Why the App is installed twice?

Most probably you have specified two activities as launcher in manifest file. This invokes this issue.

change it to

The app is not installed twice. You are not looking at apps. You are looking at launchable activities, ones with this <intent-filter>:

        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>

You have two activities with that <intent-filter>, and so you will have two activities in the home screen launcher. If you do not want both of those activities in the home screen launcher, remove that <intent-filter> from one of them.