Every human mistake is they never check what is inside their recycle bin before proceeding.
The very first thing you should do when you want to create a user is to check your Office 365 recycle bin! Why? Because is best to avoid another problem for yourself unless you are the problem. To also avoid duplication occur in Office 365.
Imagine yourself hitting the brick wall by creating and deleting repetition of a particular user account, setting yourself in panic moment and continue getting errors such as,
- Unable to get the user account to sync up to Office 365
- Incorrect user principal name
- Worst -> Incorrect Immutable ID
So ALWAYS CHECK OFFICE 365 RECYCLE BIN BEFORE PROCEEDING TO CREATE!