The root cause of this problem is an know issue of ES, please see https://github.com/elastic/elasticsearch/issues/8105. Since the index creation is async and there will be a window time that when checking if index is created or not, but there is not an official solution to this problem.
Steps to reproduce:
1) Start portal
2) Go to Felix Gogo shell
3) Stop bundle named "Liferay Portal Cluster Single" in gogo shell
4) Restart the bundle just be stopped
You would see IndexAlreadyExistsException in console, for more detailed error log, please see error.log.
Please note that this is a race condition issue, it means you may not reproduce it each time, but you still have a great possibility to reproduce it.