NSNotification Observer added in viewWillAppear is executed twice in iOS 13 - ios

Question

I came across a strange problem when handling a NSNotification iOS 13
// Adding the observer in viewWillAppear
- (void)viewWillAppear:(BOOL)animated {
[super viewWillAppear:animated];
...
NSLog(#"Adding observer");
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(handleNotification) name:#"someNotification" object:nil];
}
- (void)viewWillDisappear:(BOOL)animated {
[super viewWillDisappear:animated];
...
[[NSNotificationCenter defaultCenter] removeObserver:self name:#"someNotification" object:nil];
NSLog(#"Removed observer");
}
- (void)handleNotification {
NSLog(#"Handle notification");
}
I have double checked that the notification is posted only once. There is only one place in the app where someNotification is posted and a breakpoint and NSLog confirms, that this code is only executed once.
Result in iOS 12 and below:
Adding observer
Handle notification
Result in iOS 13 and below:
Adding observer
Handle notification
Handle notification
So in iOS 13 the observer is called twice while the exact same code in iOS 12 and blow is only called once.
My first guess was, that for some reason in iOS 13 viewWillAppear is called twice that thus the observer is added twice. However, in this case the NSLog output Adding Observer should also appear twice, shouldn't it?
BUT: The problem can be solved by removing the observer in viewWillAppear before adding it. This makes sure, that the observer is added only once:
- (void)viewWillAppear:(BOOL)animated {
[super viewWillAppear:animated];
...
NSLog(#"Adding observer");
[[NSNotificationCenter defaultCenter] removeObserver:self name:#"someNotification" object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(handleNotification) name:#"someNotification" object:nil];
}
So, since this solves the problem the observer obviously is added twice in viewWillAppear. But why?
Why is viewWillAppear called twice in iOS 13? And even more important: How is it possible, that it is called twice but the NSLog output Adding Observer appears only once?