Thank you for using Aspose.Email.
I am sorry but can you please elaborate this issue a little more. I have generated an email with the HTML body as you mentioned above in your post, and it was received fine with the text displayed only once.
We would request you to download and use the latest version of Aspose.Email for .NET 2.2.0
and let us know your feedback. If the problem still persists, please provide us with your sample project or runnable code along with the sample email message, so that we can reproduce the issue at our end and forward it to our development team for further analysis. Looking forward to assist you further.