Lambda is triggering multiple times and it sends duplicate emails

There could be two root cause of your problem.

1. SES is sending duplicate emails. Although SES is designed to send only one copy of the message but if in case it don't receive an acknowledgement from the recipient email server then it treat it as a failure and would do a retry of that message. please check for the duplicate message if the message id is same or not.
2. Duplicate invocation of lambda. first check your cron scheduler. second, review your cloud watch logs check if the duplicate invocation has same request id? if it is a duplicate invocation , use this configuration to handle the retries - https://docs.aws.amazon.com/lambda/latest/dg/invocation-async.html#invocation-async-errors

Another possibility is that your Lambda function is raising an exception after sending email, triggering the Lambda service to retry. This would be visible in the Lambda function logs which you've probably already checked though.