Android Manifest - duplicate registration for Activity but it's possible to register Service twice

When I add the same Activity twice to manifest, it's reported as error:

<activity
    android:name=".another.MyActivity" />

<!-- second entry marked with error -->
<activity
    android:name=".another.MyActivity" />

Duplicate registration for activity .another.MyActivity

But when I add the same Service twice, there is no error, why?

<!-- no error -->
<service
    android:name="just.AnotherService"
    android:exported="true">
    <intent-filter>
        <action android:name="some.ACTION" />
    </intent-filter>
</service>

<service
    android:name="just.AnotherService"
    android:exported="true">
    <intent-filter>
        <action android:name="some.ACTION" />
    </intent-filter>
</service>