Which reasons could explain the duplicate email messages?

Explanations:

Standard SQS queues guarantee at-least-once message delivery. This means that under certain conditions, messages may be delivered more than once. During periods of high traffic, this behavior can result in duplicate messages being sent to the Lambda function, which would then cause duplicate emails to be sent via Amazon SES.

Standard SQS queues do not support exactly-once processing; they are designed for at-least-once delivery. Therefore, the duplicate email messages are not a result of user error but rather due to the inherent nature of how messages are delivered in SQS.

DKIM (DomainKeys Identified Mail) configuration does not relate to the generation of duplicate messages. Incorrect DKIM configuration would primarily affect email deliverability and authentication, not the duplication of email messages.

If the SQS queue’s visibility timeout is lower than or the same as the Lambda function’s timeout, messages may become visible again in the queue before the Lambda function has finished processing them. This can lead to the same message being processed multiple times, resulting in duplicate emails being sent.

A high Amazon SES bounce rate relates to deliverability issues of emails sent, not to the duplication of emails. Bounce rates indicate that emails are failing to be delivered and do not cause duplicates.