Answer
When MailEssentials adds a Disclaimer to a HTML email, it also adds a unique hidden HTML tag. For HTML body part the following HTML tag is added to the disclaimer:
<A title="gfidiscl.xxxxxxx" href="#"> </A>
Where the xxxxx is either the SID or the domain name.
E.g.: <A title=3D"gfidisc.gfi.com" href=3D"#"> </A>
If an email is sent the disclaimer gets added with a tag. When the recipient replies and the original sender again replys to the reply - MailEssentials detects the HTML tag and won't add the disclaimer a second time.
Some mail servers or mail clients unfortunately remove the hidden HTML tags from the MailEssentials disclaimers. Therefore, if such email gets processed by MailEssentials, it would not find the HTML tag, therefore fail to detect the disclaimer and add the disclaimer a second time. This is a function of these mail servers and clients and cannot be changed to interpret these tags as expected.