CBSubs Mailer - Some duplicated email messages

We are using the CBSubs mailer to send out membership renewal reminders.
The emails are generated X days prior to subscription expiry. Fairly regularly (in about 20% of messages), the email is duplicated. The message id's are sequential. The trigger conditions are simple (See Snap attached).
What could cause these duplications, how can we best problem solve?
Regards,
Robert

The mailer is sending the email to the same user twice or you have 2 mailers that should be sequential sending at the same time? If 2 mailers please provide both of their timing configuration.

The one mailer is sending emails twice.

We have several Mailers prompting members to renew, 35 days, 20 days and 5 days. Each mailer has identical settings other than the "days prior", each mailer has a similar heading containing the "days prior" (e.g. ATAA: Overdue Subscription Notice - 20 days ). The diplications defnitelt occur within the same mailer. Similar duplications are occurring in the other mailers but again for only around 20% of the batch. We can find nothing unusual about the members who are getting duplicated emails.

Regards, Robert