The new version of MailShot and MailShot Pro addresses something only discovered recently, which is that some providers do not accept any email with an address repeated. Error messages from the providers never really explained the problem, and it was only detective work by a user which revealed it.
This could happen if, for example, a couple use the same email address with different names.
Mailshot already takes steps to prevent you adding an email address again, but there was a way you could add them both at once using the Contact Checklist.
The new update prevents you adding the same address in this way, and scans your groups to fix any cases where this has already happened.
* Yes, this is a Star Wars reference