I have a straightforward setup here:
User A forwarder forwards to B and C.
When user C receives an e-mail and A has been CCed, two e-mails are delivered to user C. Up to 5 copies are being delivered to User C because of forwarders and CCs doing their combo dance.
I recently change web-hosts, On my previous host, the duplicates never arrived. I have no clue what changed in the new host's cPanel.