Issue
When using the Update Document action, content within Rich Text fields are duplicated.
Resolution
Upgrade to the latest build of Nintex Workflow. More for information on this process, see Updating Nintex for SharePoint - What you need to know.
Additional Information
This was a known bug within Nintex Workflow for SharePoint. This has been fixed in build 3.1.9.0 for Workflow 2013, and build 2.4.9.0 for Workflow 2010.