Short answer
HubSpot's KB: you can't connect a Messenger account to more than one HubSpot account. Facebook's API doesn't share visitor emails, so duplicate contacts can appear if someone chats on multiple pages. Connecting channels needs Account Access permission.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Messenger chatflows can't be moved from the inbox to help desk; reconnect instead.