External tag - double tagging problem?

Question

I have an additional policy rule in place to tag all incoming email with an external email notification tag in the subject line. Sometimes when replying to all external users that have been CC'd, and then receiving their reply to that email, subject lines are being double tagged. In other words multiple tags are appended to the subject line. Is there any way to stop this multiple tagging problem? 
 Additional notes: 
 The multiple tagging problem does not occur when emailing with single recipients 
 
 Thanks

MassimoForni · Answer

I don't think it's possible, sorry. 
 You can set or replace a header the way you want, but not subjects