Agent saving mail sends them again!

In an application a background agent edits mail documents pasted from the Inbox, to change the form and add some new fields. The documents are then ComputedWithForm and saved. Sometimes -but not always- the pasted mail gets mailed out again to all recipients, which is very frustrating.

Why is this, and how can it be prevented???

Subject: Look for a MailOptions field on the documents that get sent

Subject: Indeed!

But how come this field is set to one on some mail and non existent on others?