If your customers are receiving two email receipts, one from you and one from Authorize.net, use this tutorial to disable the Authorize.net email receipt.
First, log-in to your secure Authorize.net merchant panel
Once you have logged in, you will need to go to your settings page
In your settings page, select Email Receipt
In your email receipts page you have the option to disable customer email receipts
Unselect the checkbox and submit
And you’re done!