Issue with users copying tasks

Hello, 

We bill our clients monthly by using Fusion to sum the planned revenue of all tasks completed in the previous month (with some other criteria). The scenario then adds '666' story points to those tasks which essentially says they have been 'billed' so they don't then bill again the next month. This was set up when my company first moved over to Workfront.

A few months ago I noticed some projects weren't billing correctly and it seems that tasks that have previously been billed (and therefore have '666' story points assigned) are being copied within projects, and so weren't billing again when they needed to.

We've communicated this out to our users asking them to not copy tasks, but it still seems that this is happening each month. I have a report that I review each month that shows me the tasks that have '666' and were completed in the relevant previous month (and therefore shouldn't have '666' on them), but there is the chance that the tasks have been completed, billed, and then re-opened and completed again. I can check this through system updates, but wondered if it's possible to check if a task has been copied and what the original task was - so I can pick up particular scenarios with users (before possibly accusing them of copying tasks when they may not have done so)?

Alternatively, is there a way of preventing tasks being copied so I can prevent this from happening in the first place?

Thank you

Lana