Yes, end users can submit more than one proposal for your event.
Once the first submission has been completed, the end user will need to navigate back to your submission form link to complete a new submission. The end user will not be able to complete a new submission from the portal.
Additionally, completing more than one submission will result in duplicate contacts being created if the speakers for each submission are the same.
Our recommendation is for the event admins to complete an audit once the submission form closes to remove duplicates and ensure all submissions have speakers assigned to them.