Hi LJulien1,
Welcome to Microsoft Community and appreciate your time taken for sharing detailed information. I understand that Adding new user with same first five letters in email address to existing user. I am glad to offer you help on this issue.
In our tests, it makes a difference. Windows 10 uses the first five letters to create the user directory. After User 1 has been create, the first five letters are the same. However the second one would be rober_.... The following( ... ) may be random characters generated to distinguish between previous account names. If you don't like the name, you could change your user name yourself.
Click File Explorer > This PC > C disk > user > your account profile name > right click on it and then change it into what you want. (But be sure to log out of your account, then log in to your administrator account, and then go to the appropriate path to change your account name)
When you log in to your PC, the user interface displays your account name. You could log in to your account (e.g. ******@hollywood.com) from the web version, view your account name, or you could change it.
I hope the information provided is useful for you. If you have any additional questions, please do not hesitate to reach out to our support.
Best Regard,
Kevin | Microsoft Community Support Specialist
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