Symptoms

I have an activity consisting of multiple destinations set for plan all at once, create a slot for each destination. the idea is for each participant to approve in parallel and complete the activity when all participants have actioned their task. There are also escalation emails to remind participants of their need to action the task. however users are being included in the email after they have actioned their task so no longer have an active worklist item. Is this the expected behavior?
 

Diagnoses

This is expected behavior and, in your case, it all comes down to how you are specifying the escalation recipients.
 

Resolution

I ran into a similar scenario in the past and below are the detailed steps of how I configured my workflow design to accomplish this:

On the Activity, go into advanced mode for the destination rule and make the following changes(you already have the first two steps covered):
- Plan per destination | All at once
- Create a slot for each destination
- Resolve all roles and groups to users

On the Client Event, reconfigure the Escalation as below:
- On the Email Settings page, check 'Specify' next to Recipient(make sure both 'Destination User' and 'Originator' check boxes are unchecked)
- Open the context browser for the To field, go to the Workflow Context Tab, and drill down into Activity Destination Instance and select the User email i.e. 'ActivityInstanceDestUserEmail'.
Now only those destination users who have not completed their task by the escalation time will receive an email. Here a quick video walkthrough for reference: http://screencast.com/t/N4LhMdGgHHSm

A key thing to remember here is to use the Activity Destination Instance user and do not just click the Destination box for the To field. If you click the destination option, an email will be sent to every member of the role, even if they have completed their task.