The explanation for why you may intermittently notice two comments on a ticket update via Maileater.
Why is it that sometimes two activities or comments get logged when a ticket is updated via Mailteater?
Whenever a user updates the ticket via Maileater, activity log of that particular ticket will be updated. Whether it might be updating 'priority', 'Description', 'Summary' or any field, activity log will be recorded so that Analyst will know that it is updated via Maileater.
Basically, if you log a comment through web UI it always adds the comment to the activity log. So whenever log comment is added through maileater, it updates activity log this the first log comment you observe. So after that maileater updates activity log that an update is made via maileater.
Now send an update to maileater to change the priority of a ticket. In this case, observe that only 1 entry will be added to the activity log(updated via EMAIL interface). In the above case as you are adding a log comment, first it will update the activity log and then Maileater updates that there is an update via Maileater. That is the reason why we are able to see two comments in the activity log. But both the comments are different. So whenever any update occurs to a field through a maileater, activity log will be updated with 'updated via EMAIL interface' text. This functionality is working as designed.
An Idea (Enhancement Request) can be logged for functionality to be reworked in future releases.