NotificationHandler gets called twice

Hello,

unfortunately, I´ve discovered that the NotificationHandler is getting called twice. I´ve read a few articles on the web but the problem is still not fixed.

I am using the following versions:

1. MediatR 10.0.1
2. MediatR.Extensions.FluentValidation.AspNetCore 2.0.0
3. MediatR.Extensions.Microsoft.DependencyInjection 10.0.1

This is how I register MediatR

            services.AddMediatR(typeof(IInstaller).Assembly);
            services.AddValidatorsFromAssembly(typeof(IInstaller).Assembly);
            ValidatorOptions.Global.LanguageManager.Culture = new CultureInfo("de-DE");

The type ‘IInstaller’ is in the corresponding project where my NoticationHandlers and RequestHandlers are.

This is how I publish a Notification

    public abstract class DomainEvent {
        public Guid EventId { get; set; }
        public bool IsPublished { get; set; }
        public DateTimeOffset DateOccurred { get; set; }
        protected DomainEvent() {
            EventId = Guid.NewGuid();
            DateOccurred = DateTimeOffset.Now;
        }
    }

public class UserStateChangedEvent : DomainEvent {
        public ApplicationUser ApplicationUser { get; }
        public UserStateChangedEvent(ApplicationUser applicationUser) {
            ApplicationUser = applicationUser;
        }
    }

private async Task DispatchDomainEventsAsync(ICollection objectsToSave, ICollection objectsToDelete) {
            IEnumerable<DomainEvent> saveDomainEvents = objectsToSave.OfType<AggregatedRoot>().Where(x=> x.DomainEvents != null).Where(x => x.DomainEvents.Any()).SelectMany(x=>x.DomainEvents);
            IEnumerable<DomainEvent> deleteDomainEvents = objectsToDelete.OfType<AggregatedRoot>().Where(x=> x.DomainEvents != null).Where(x => x.DomainEvents.Any()).SelectMany(x=>x.DomainEvents);
            IEnumerable<DomainEvent> concatDomainEvents = saveDomainEvents.Concat(deleteDomainEvents);
            foreach (DomainEvent domainEvent in concatDomainEvents) {
                domainEvent.IsPublished = true;
                await _domainEventService.Publish(domainEvent);
            }
        }

public class DomainEventService : IDomainEventService {

        private readonly IPublisher _publisher;
        public DomainEventService(IPublisher publisher) {
            _publisher = publisher;
        }

        public async Task Publish(DomainEvent domainEvent) {
            await _publisher.Publish(GetNotificationCorrespondingToDomainEvent(domainEvent));
        }
        
        private INotification GetNotificationCorrespondingToDomainEvent(DomainEvent domainEvent)
        {
            return (INotification)Activator.CreateInstance(
                typeof(DomainEventNotification<>).MakeGenericType(domainEvent.GetType()), domainEvent)!;
        }
    }

    public class UserStateChangedEventHandler : INotificationHandler<DomainEventNotification<UserStateChangedEvent>> {     
        public async Task Handle(DomainEventNotification<UserStateChangedEvent> notification, CancellationToken cancellationToken) {
            //DoStuff
        }
    }