I can't even wrap my brain around this.

I have secondary email account on my cable service. I get two emails last night from accounts-noreplygoogle.com saying I've initiated a password change and to click the link to verify.

It gives an email address that I do not remember creating in google accounts but it IS a username that I have used in the past in other online user profiles. I'll call this "usernameB".

I figure, okay, I suppose it is possible I used "usernameB" when checking out some google service such as Blogger. I'll go to gmail homepage and try to log into it and see what comes up!

I type in "usernameB" and I think of a password that I may have used and come up with a generic password that I have reused on sites I know I won't frequent often and wouldn't you know it, it works. It logs me right into my MAIN gmail account that has a DIFFERENT username and a DIFFERENT password.

I've combed through Account Settings and can't find how to delete this second door to my account. Anyone ever hear of this or know something I can try to stop it?

You can't delete alias. Many has asked about it but I don't think Google will allow it due to security reasons.