Hi ,
According to your description, your report subscription running twice if not completed within 30 minutes, Right?
If you have one subscription failed to send at the proper time and it have delayed, it may be send at the next time.
Does the report server run in SharePoint integrated mode or native mode?
If in SharePoint integrated mode, it might be related to the recycling of the Application Pool of the SharePoint site.
If in native mode, it might be caused by the lock caused by the concurrent processing of subscriptions and notifications.
When a scheduled operation is defined, the report server creates a corresponding job in SQL Server Agent and runs the job on the date and time specified in the schedule. If somebody run the subscription manually after the job executed, the users can got two emails, so please make sure the subscription isn’t run manually during the time by someone.
The SQL Server Agent service calculates an incorrect NEXT_RUN_TIME value for the job. Therefore, the job does not always run at the correct times. This issue occurs because an error causes a date value to be changed unexpectedly, the date value is used to calculate the NEXT_RUN_TIME value. If you got the subscription execute twice many times, please edit reset the subscription.
Regards,
Joy