Hi Amanda,
Thanks for posting your question! Within the site designer, you can click on the sessions widget and you'll see a setting on the right hand side that says "Allow Registrants to Select Overlapping Sessions". You'll want to make sure to toggle this off so that they can't select two sessions occuring at the same time.
Please let us know if you have any additional questions!
Thank you!
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Erica Wise
Senior Client Success Manager
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