Potential fix for wrong notification order on slot change

src/main/java/de/webalf/slotbot/model/Event.java

@@ -309,6 +309,10 @@ void removeSquad(Squad squad) {
 		getSquadList().remove(squad);
 	}
 
+	public void unslotIfAlreadySlotted(User user) {
+		findSlotOfUser(user).ifPresent(oldSlot -> oldSlot.unslotWithoutUpdate(user));
+	}
+
 	/**
 	 * Informs the event about an slot update (slot, unslot, new slot(s) created, slot(s) removed).
 	 * Uses {@link Event#moveReservists()} to change reserve if needed

src/main/java/de/webalf/slotbot/service/EventService.java

@@ -257,6 +257,8 @@ public Event slot(long channel, int slotNumber, UserDto userDto) {
 		Event event = findByChannel(channel);
 		Slot slot = event.findSlot(slotNumber).orElseThrow(ResourceNotFoundException::new);
 		User user = userService.find(userDto);
+		event.unslotIfAlreadySlotted(user);
+		eventRepository.saveAndFlush(event);
 		slotService.slot(slot, user);
 		return event;
 	}