JQL Filter to show only the 1st substask of each parent

Hello, 

My team is using BR issues and subtasks to follow project deployments.

To ease the team members dailywork , I've created a dashboard to show each opened subtask assigned to current user based on the following filter:

project = XXX AND issuetype = Sub-task AND status in (Submitted, "Resolution In Progress", "Task Postponed") AND assignee in (currentUser())

But I would like to go further by showing only the 1st opened subtask of a given parent in order to help people concentrating on their most urgent task of each project.

Thus I would like to move from a result like : 

subtask 1 ( parent A)

subtask 2 (parent A)

subtask 1 (parent B)

subtask 2 (parent B)

to 

subtask 1 ( parent A)

subtask 1 (parent B)

Any  suggestion?

Thanks for your help