Was wondering is there somewhere else associated with my Mail program that utilizes my email address?
Yes. You're on the right track. Apple's "account already exists" message is an example of user interaction such that — while technically correct — is entirely unhelpful.
The solution is to "sign out" of every service that uses that email address. Those services are likely to include Contacts, FaceTime, Messages, Mail and others. You can use System Preferences > Internet Accounts to examine each one of the (possibly) many accounts you have that are already using that email address. So that's what it means by "already exists"... because it does. Of course it does. It's just not helpful because you want to add that already existing Account to Mail.
A quick method to rule out all the Apple services that rely upon that Account is to "sign out" of iCloud altogether. Then, add the email account again. Sign in to iCloud when prompted, and select the services you wish to use with that iCloud Account.
For reference,
Remove email accounts in Mail on Mac - Apple Support
Add email accounts in Mail on Mac - Apple Support