I've been looking at the documentation on the [`inbox_item`](https://api.stackexchange.com/docs/types/inbox-item) type, and I was wondering what is the correct way to uniquely identify an inbox_item? 

Is the "link" URL enough to uniquely identify an "inbox_item"? Is it ever the case that two links will point to the same notification event? 

What happens when questions are migrated to new sites? Does the link change in this case too?