The MailChimp system sends only one campaign at a time to each subscriber's address, but a few circumstances may result in multiple deliveries. Multiple deliveries typically affect a small handful of subscribers—not the entire list.
In this article, you'll learn what can cause duplicate messages.
Subscriber Has Multiple Addresses
Sometimes subscribers sign up for a list more than once, with different email addresses. Subscribers with multiple aliases may forward all their mail to a single address to manage their inboxes. If more than one alias is on your MailChimp list, the subscriber could receive duplicate messages.
To resolve this issue, search your account for your subscriber's name. If the subscriber has more than one address on the list, ask for their preferred email address and delete the extra addresses.
Campaign Was Sent More Than Once
If you send the same campaign to List A and List B, a subscriber who's on both lists will get the campaign twice. We recommend managing one list of subscribers to reduce the chances of this happening.
To resolve this issue, combine lists into a master list, and use groups or segments to target subscribers. A campaign that goes to several groups or segments of a list is only delivered once.
Listserv Addresses on Your List
If a listserv address is on your list, your subscribers who are also on the listserv might get duplicate messages.
To resolve this issue, delete listserv addresses from your subscriber list and send to individual recipients only.
Subscriber Server Delivery Issue
If several subscribers from the same domain, like Mary@CompanyA.com and Joe@CompanyA.com, receive duplicate campaigns, there could be a delivery issue at their server.
To resolve this issue, ask subscribers to contact their email provider or IT department to troubleshoot.