When using the attachments feature on the User service, you may have encountered a situation where attachments are being duplicated at each step of the process. This will happen whether you reference the “list of document” variable itself or the XPath expression of the variable.
The way to get around this issue is to reference the XPath expression of the variable and append  to it.
In Workbench, navigate to the Attachments tab of the User service.
In the Output Attachments section, make sure that the dropdown is set to XPath expression.
Select your variable of type list of document and append  to it.
It should now appear as /process_data/myAttachments.
Make sure you use this same notation (/process_data/myAttachments) on each User service step.
Original article at http://blogs.adobe.com/ADEP/2011/08/stopping-duplicate-attachments-in-workspace.html.