I have a client with an email address like clientname@clientDomain.com. I can log into his Microsoft 365 account with his email password with no issues.
However if I use the same address when setting up a new computer the password is wrong. I was able to "reset" the password and get the new computer setup. Now it appears he has two accounts at Microsoft under the same email address. Not a big issue except his Microsoft 365 account includes the Office desktop subscription. When attempting to activate the desktop version of office Microsoft does not recognize he has a subscription to Office and is requiring that he buy it to get activated.
This experience leads me to believe there are two different accounts at Microsoft that have the same email address.