security or privacy breach at Microsoft

I find this hard to follow. Are you saying there are two Skype accounts with the same or different gmail address now ? You can visit the Skype page for each gmail to see if the profile info matches and get a clue as to how the new account was created. That is the one I would delete since your Win10 signs in with your orig account.

I believe, if you sign in to your PC with an MS account, the Store Skype App will sign you in with your MS account by default. You can always sign out then use an other Skype account.

"years old original Skype account user name is "first.last". The new unauthorized Skype account user name is "coincidentally" my Microsoft Account user name which is just a unique string and when I created the Microsoft Account I used my gmail address as the string so it is "first.last@gmail.com"

If both account sign-ins are the same gmail.com address, there's no issue. By 'User name', are you referring to the name next to your avatar or the sign-in name? The other issue is which version of Skype you are using, the new Store app or the 'classic' Desktop. Here's a screenshot of what my upper L corner looks like in both versions. The L grab is from Desktop, the R from modern Skype app (no name next to avatar but click avatar, you see User name and email). They have the same sign-in email and the same User name next to my avatar if I Skype somebody. Post yours to clarify further.

When I say "user name" I mean the login name or sign in name.

H2ONaCl said:

When I say "user name" I mean the login name or sign in name.

Each Skype account has an email address associated with it, PERIOD. Even if you use your Skype alias to sign-in instead, it still has an email account tied to it. Sign out and then sign in with just email. If it's the same for both, there is no issue.

Again, which version of Skype are you using ? Skype was trying to un-bundle accounts a while back. I had a Facebook sign-in which I had to consolidate with my MS email so now I only have one. But the Facebook log-in had the same avatar, personal info, etc. I was using both to keep track of which version I was using.

The following is me having a conversation with myself using my LiveID that MS assigns to the Skype Store App automatically and the Desktop Skype with my Skype ID from years ago...

Windows 10 has Skype version 11.18.596.0. It doesn't matter which version of Skype I am using. This is a dual account problem; not a version of Skype problem.

Yes, the contact details has my email address (and it is the same in both accounts) but this is not about email addresses because that is merely contact details. This is about dual login names. The fact that one of the login names is an email address has nothing to do with the question. A login name is just a unique string. The fact that the string is an email address is not an issue.

The issue is that an unauthorized account was created and I want to know the implications of closing the account that is tied to Windows 10.