In the attempt to create or invite a new user with email address "firstname.lastname@example.org", the message appears "The user id is in use. Choose another email." even though the user was already deleted from the other instance it was being used in.
This message can appear as at the time the user "email@example.com" was being created or invited to another instance, after deleting "firstname.lastname@example.org" from another instance, the user was not completely deleted from the other instance. At times it can take about 5-15 minutes for a user to be completely deleted from the system, so allow some time before re-creating the user.