Still a single copy if all those users are on the same mailstore. Your not moving the blobs around, simply the pointers to them.
In a standard ZCS/postfix install, if you send a message to a distribution list and all of those sers are on the same MBS server, postfix creates a single LMTP transaction to deliver them. You get one blob file with hardlinks for the different users, one copy in the redologs, ie message is written at most twice.
When you share a folder with a user on another mailstore the mountpoint is just accessed remotely, still one instance. Unless that user on mailstore2 drags/moves/copies an email from the share (on srv1) to one of their own folders (on srv2) - then you have a new blob.
Zimbra doesn't use any traditional mailstore like mbox or maildir. It uses a proprietary file-per-message in a hashed-like dir hierarchy linked to a MySQL database for metadata, so it is much more efficient than either. Each message/attachment/etc is represented by a file blob. Checkout
Account mailbox database structure - Zimbra :: Wiki for more info. As discussed above, we also make use of hard links for identical messages that come in at the same time/ID to multiple recipients if the accounts are on the same mailstore (aka single instance storage). I seem to remember SMTP clients who hit default_destination_recipient_limit of 50 (ie: split up delivery of larger messages into new sessions when sending), or when they get 450 error 'try again later' upon hitting the receiving servers smtp_recipient_limit/smtp_recipient_overshoot_limit (1000), end up creating a new blob (have to check if it's also a new message ID).