Requirement:

Customer want to create a user in their account but system says it is already in use in another account.


Solution:

Administrators have a functionality called “Mark User Obsolete”. This action is applicable only on deactivated users. 

This will internally rename a user a@b.com to a@b.com.<orgid>.obsolete ;  

It allows you to use a@b.com in another tenant. An obsolete user can again be activated only be removing activate tag.