Attachments binary in document

When attachment is added to a document, if the same attachment added multiple times, I believe, CB only keep one set of binary.However, if we have multiple documents and same attachment is being attached various documents, Would CB still keeps one copy for all? OR this is only valid for per document?

Yes. We store blobs content-addressably: There is only one copy, in any database, of all blobs with the same content.