Usually, the duplication happens when the task list has two triggering events, the most common is when the task list is set to be triggered automatically by an anchor date, in this case, the start date, and is also triggered via the new hire flow.
Could you please check if that's the case? Go to Settings > Task lists and see if the task list has an anchor date
Next, go to Settings > Flows > New hire > Click on 'manage' in the relevant flow > Features, and check if the task list is also associated with the flow.
If that's the case, you will need to remove one of the triggering events, this will fix the duplicating tasks issue.