Why are some recipients receiving multiple copies of my Campaign?

The system does not send out duplicate Campaigns on its own.

Generally the reason recipients would receive duplicates is because they are actually stored in your distribution list more than once (e.g. they have a personal email address setup to forward to a business address, and both of those addresses are in your distribution list). In other cases, if recipients are receiving Campaigns from multiple senders at the same company, they may not realise how many different messages they are actually receiving.

If both of these causes have been ruled out, the recipient's mail server may have an issue. If for some reason it is not registering that the email has been delivered to the inbox, it may keep trying to deliver (which to the recipient would look like they are receiving the Campaign multiple times).