There's a saying that the word obfuscate obfuscates - and it does; and if you're wondering what this is all about, well, this is the exact situation that Microsoft has created by consolidating the access portal for many of their online services where you can use either your personal or work Microsoft accounts. To add to this mix, you can use your work email address to create a personal account.
But why would this be an issue? Possibly, one of Microsoft's services, such as Skype, may have been utilised - and this would have required a personal Microsoft account. As such, you likely would have used your work account in order to create the Skype account. If you're on Office 365, this would then mean you have both personal work accounts with the same login, namely your work email address.
More recently, Microsoft is making the distinction more clear, such as in the example below, if logging into web services like OneDrive, it clearly gives you the option to sign into the correct account.
Mobile devices are a different story, and apps may not be updated to make the distinction. To avoid confusion in regard to this ensure that you use different passwords for both accounts.