Adaptive Subscriptions: Duplicate order confirmations sent on future billings

When using the Adaptive Subscriptions extension you may encounter a rare situation where after a customer submits a new subscription order, future billings of that subscription appear to send out duplicate order confirmation emails from your store. If this happens you may be encountering a rare condition involving your servers configuration or SMTP relay.

Common Solution

To resolve this, it's likely that all you need to do is navigate in the Magento Admin panel to Stores->Configuration->Sales->Sales-Emails->General Settings->Asynchronous sending, and set this option to "Enabled". Once enabled, your Magento store should begin sending emails in a more reliable manner.

Further Investigation

For any cases where this configuration setting does not resolve the issue, you will want to review your SMTP relay or other aspects of your email processing or server configuration (such as multiple cron tabs running in more complex server setups). If you happen to run instances of your store on multiple servers, you will also want to review that stores sharing similar data are not processing orders at the same time; you can usually confirm if this is the case by reviewing the headers of the duplicate emails.