"The inbox stream contains all activities received by the actor." (#ActivityPub Rec). However, AP/AS2 collections (including "special" ones like Inbox, Outbox, Followers, etc.) do not contain Objects or Activities. They contain URI *references*. That's why one Create/Note can be referenced by many inboxes. It may look like Collections contain Objects because of typical server JSON-LD serialization, but don't be fooled. It makes a difference for data lifecycle management and storage models.