Google Workspace "send to" was also sending to me

The problem

I recently switched from a free google account to a Google Workspace Business Standard account.

It's just myself and one other user on the account.

I found that whenever replying to the other person's email, the "reply to" field had my own email address in it instead of his, and if I hit send, it was sending to both.
After some googling, I discovered it's about a hidden box that was checked in my own email settings called "alias."

Here's what I did to fix it but I don't know how I figured this out. All I knew from googling was to looking for the word "alias." I had found these words: "These issues occur because Gmail treats your Send mail as address as an alias of your primary address. You can change this behavior in your mail settings by deselecting Treat as an alias."

Here's another link if it helps you further: Google Workspace Help

The solution in words

1. Go into my gmail account
2. Settings gear --> see all settings --> accounts
3. Find the section that page that says "Send mail as:"
4. There were two email addresses there. Mine was the default. His was the other email. 
5. Because mine was the default, that really made it look like it has no effect. I was wrong
6. Out of curiosity, in his email, I clicked in "edit info" and boom...I saw that the box was checked for "treat as an alias."
7. I unchecked that box.
8. All good. Now when I hit reply to this person's email the reply to has his email address and not mine

The solution in pictures

Here is what I found in send mail as:

And when I clicked edit info on his email, I saw this box checked

But after unchecking the box, now notice it says "not an alias" under his email address