Hi Katie -
You may want to check your Event Emails to see what you have Active and Enabled. There may be a "thank you for submitting your registration email" enabled as well as a "Registration Pending" email. Based on this screenshot, it shows that two emails were sent, but it doesn't necessarily mean that it was the same email message.
You can also try running a "Sent Email Details" report to confirm exactly which emails were sent out:

Hope this helps!
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Rebecca Yousif
Senior Administrator - Event Technology
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