Are you sure another system hasn't already Prepended?
If you don't want the ESA to add multiple Prepends, then remove them from the config. There are the built-in options to Prepend Subject.
Such as Anti-Spam, Anti-Virus, Graymail etc.
But to avoid any duplication, then within these areas you can add an X-header and use this in a logic rule to decide on Prepending once.
( note on security: first create a Message filter via CLI - not Content Filter as too late - deleting the header you are going to use to avoid others injecting messages that match your rule
Then have a Content Filter on each policy that removes these internal headers - so you are not advertising them to the public )
Use the X-header in a filter or content filter to Prepend the subject.
That way you can use some logic to decide on what to Prepend and only Prepend once.
Although, usually if the Prepends are unique then that simply tells the user that it failed on multiple parts and becomes more threatening and tells the story.
Suspect Spam: AV_Unscannable: Marketing_Bulk:
