Two Duplicate User Names on Windows 10 Welcome Screen. Why and how to fix?

How The Issue Looks Like

I’ve got two exact same user account names on my welcome screen. I don’t know, what’s the difference between them and which one I should use. Why the duplicate names emerged and how to fix the issue?

The Cause

Here are all possible reasons why duplicate user names can emerge on the welcome screen of your Windows 10:

1. You have changed the name of your computer after you enabled auto-login.
2. You have changed the password for your user account after you enabled auto-login.
3. You have switched to Microsoft account from the local account or vice versa.

How to fix two equal User Accounts and Leave Only One of Them

1. Disable auto-login.
   Press Win+R > type netplwiz > hit Enter.
   Set the checkbox Users must enter a user name and a password to use this computer: 
2. Ensure your machine has the desired network name. Rename it right now if you wish: 
3. Enable auto-login again if you really need it. Type the correct password and its confirmation: 

Note that you will have the issue again if you rename your PC while auto-login is enabled.