How do I transfer my mac mini data to a new mac mini?

Because you used Migrations Assistant. You should have used Setup Assistant during the setup process. See:

Re: Migration assistant created a new user when transferring files from a PC to mac

The above apples to migrating from a Mac via Migrations Assistant

That's because you used Migration Assistant after you setup your new MacMini. The trick is to use Setup Assistant when you setup your new MacMini.

You can't use Migration Assistant to migrate data to an existing User Account. It Migrate your old User Account.

Your options now:

1.Delete the 1st new User Account that you created when first starting the new MacMini. You would do that while signed into the older User Account. Job done.

2. Erase the drive and reinstall OS X, and when prompted use, Setup Assistant. This would take a lot of time, and you would wind up with the same configuration as option 1.

You sign on to the account that you migrated from the old MacMini. That would be the one with all of your old data.

Then you go to System Preferences, Users & Groups, click on the pad lock, enter your password, then highlight, under Other Users, the account that you created when you started created with your new MacMini, and press the "-" button.