User Sync is Duplicating the Users

I am having the same exact issue.

When a subscriber updates their email address in the wordpress interface, it creates another entry within Mailchimp with that new email, instead of changing their existing subscriber email address within Mailchimp.

This issue is discussed with more technical detail on the plugin project’s issue tracker. The gist is:

1. Until recently, MailChimp provided no means of updating an existing subscriber’s email address programmatically. ??
2. The best solution available for most people was to simply subscribe a new address, so as not to erase historical data. This is what the plugin did.
3. Recently (sometime between October 2016 and January 2017), MailChimp finally added the ability to update a subscriber’s email address, but no one noticed because they didn’t say anything about it.
4. I looked into the issue tonight and discovered that MailChimp allows this now, so I wrote an unofficial patch to add this capability to the plugin.

Advanced users can manually fix this issue in their copy of the plugin by applying this diff.

For intermediate users for whom this is a high-priority issue (i.e., you do not know how or cannot manually apply the linked fix), then you can download a fixed version of the plugin and replace your existing plugin with the fixed version by manually installing it, until the plugin author releases an official fix.

Hope this helps.

To add to Meitar’s comment, we’ve just integrated his proposed changes into the plugin so that it can now properly update a subscriber’s email address without duplicating the subscriber in MailChimp.

Let us know if you have any issues with version 1.5.3 please.