When a customer sends an email to the Email-to-Case address, two cases may be created instead of one. This issue occurs in both Salesforce Classic and Lightning Experience.
Email-to-Case is a Salesforce feature that automatically creates a Case record when a customer sends an email to a designated support email address. Duplicate cases can result from misconfigured Salesforce automation (Process Builder, Flows), incorrect email server forwarding settings, or emails being sent to both the customer-facing address and the internal email service address simultaneously.
Note: Check whether "On-Demand Service" is enabled, as this affects how emails are processed.
See also:
Defining Email Service Addresses
Configuring Routing Addresses for Email-to-Case and On-Demand Email-to-Case
Bounce back loop from 'Email-to-Case' creates more than 1000 Cases
Disabled Ref Id and New Threading Behavior for Email-To-Case
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