To create a subtask only if one does not already exist for the respective issue. If subtask doesn't already exist for this parent issue, then only it will create a new subtask else it will not generate the subtask at the time of transition of the workflow.
Instructions
Steps to create a post-function for this scenario.
- Go to the Cogwheel and click on Issues.
- Under Workflows section select the workflow to add the post-function.
- Click on Edit link and select the transition steps to add the post- function.