How can I stop a field to appear multiple times in an auto sender email?

You have to edit the notification/autoresponder and remove the duplicates by deleting the row(s):

If the email template is not formatted, you may consider recreating the whole setting. Delete the existing email and create a new one.

Here's how to delete: https://www.jotform.com/help/132-How-to-Delete-Notifications-Autoresponder.

To create: https://www.jotform.com/help/25-Setting-Up-Email-Notifications.