Hi wadefromglenwood springs,
I understand that it can be quite annoying when duplicate emails are sent to your recipient, when you only intended one to be sent off. In my previous experience with this type of issue, it is usually caused by an alias email address being used along with the original email address. In this case, if you are using the Mail app on your Mac and have both the hotmail.com address and it's alias as a separate account. When emails are sent from either the primary or alias address, both accounts will send the same email.
To resolve, you should remove one of the accounts, whether primary or alias from the Mail app using the steps in the following support article: Mail (El Capitan): Set up Mail with your email accounts You can also disable them temporarily so when sending an email from either account, the other should be disabled while sending. This should alleviate the duplicate emails being sent off.
Thank you for using Apple Support Communities to post your question. Have a nice day.