The agreement is pretty well covered in legal terms. Here’s a little suggestion to facilitate better control over repeat/clash of topics.

Create a page dedicated to the topics covered under each event, and within each event is a full list of speakers and their respective topics. If you can, include a point-form introduction of each topic covered by the speakers. This is to further allow potential applicants to browse through and checked if their topic had been previously covered prior to their submission of proposal easily.

This might be able to prevent or at least, minimize the possibility of someone taking great pains to prepare a full presentation only to find out it has been rejected as replica topics.