Need some help with Document Variable in a Multi-instantiation Task

Hi
I am using Subscription version 7.1.2
I have a Task which needs to be performed in parallel by multiple users.(Multi-instantiation Task)
The users who perform the task needs to upload a document each .
I have declared a Document variable in the Process Level .
I have defined the Document as a Contract Output. The document variable saves the Contract output .
This works only when 1 users uploads a document.
But if 2nd and 3rd users also uploads documents , then the variable gets overwritten every time.
It saves the document value which was added last .
The first and the second user uploaded documents are missing.
I have tried to use java script to Append the value of previous document with Current document .(Since the variable is essentially a list) but I got error.
I have tried to define it at Operation level as well using script option to append the contract output in to the Document variable but I am getting error as well.

Any help or guidance would be much appreciated.