Firebase Screen Tracking calls “screen_view” event twice

Question

I try to use Screen Tracking feature of Firebase Analytics.
Here's the code of a screen in my app. I'd like to track transitions to this screen, and set the original screen name in viewDidAppear.

import UIKit
import Firebase

class Section1412: UIViewController {

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(true)
        Analytics.setScreenName("Sceeen1.4.1.2", screenClass: "Section1412")
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

But Even though I made screen transition only once, screen_view event is called twice.

1st:

2017-10-25 20:16:49.109820+0900 iOSTestApp[14863:709645]Firebase/Analytics][I-ACS023072] Event logged. Event name, event params: screen_view (_vs), 
{
    firebase_event_origin (_o) = auto;
    firebase_realtime (_r) = 1;
    firebase_screen_class (_sc) = Section1412;
    firebase_debug (_dbg) = 1;
    firebase_screen_id (_si) = 6534403927247648702;
    firebase_previous_class (_pc) = NativeTableViewController;
    firebase_previous_id (_pi) = 6534403927247648701;
}  

2nd:

2017-10-25 20:16:49.115297+0900 iOSTestApp[14863:709645] [Firebase/Analytics][I-ACS023072] Event logged. Event name, event params: screen_view (_vs),
{
    firebase_event_origin (_o) = auto;
    firebase_screen (_sn) = Screen1.4.1.2;
    firebase_realtime (_r) = 1;
    firebase_screen_class (_sc) = Section1412;
    firebase_debug (_dbg) = 1;
    firebase_screen_id (_si) = 6534403927247648703;
    firebase_previous_class (_pc) = Section1412;
    firebase_previous_id (_pi) = 6534403927247648702;
}  

Both are called within about 1 sec. 1st has no firebase_screen key, and 2nd has the same class name in firebase_previous_class key.
Is it a Firebase specification? Or is there any mistake in my code?

Answer 1

From Firebase team: This case would happen for view controllers that contained other view controllers (parent-child relationship) so there is nothing wrong with your code.