Hey Joanna,
Thank you for reaching out to us!
I’m afraid we don’t have a built-in option to prevent more than one event from being submitted in the same place and time.
If you want to control what events will be displayed in your calendar after they are created via Community Events Submission form, I would suggest you to change the “Default status for submitted events” (under Settings > Community > General) from “Published” to “Draft”: this way you can approve/control what will be published or not.
I hope this helps! Let me know if you have any other questions and I’ll be happy to assist!
Best Regards,
Patricia