In uploading users, there are four options for Upload type.
The help says this:
Updating existing accounts
By default Moodle adds new user accounts and skips existing users lines where the
username matches an existing account. Set "Upload Type" to Add new and update existing users, and existing user account will be updated.
- Add all, append number to usernames if needed
- Add new and update existing users
- Update existing users only
Warning: errors updating existing accounts can affect your users badly. Be careful when using the options to update"
What exactly does the "Add new and update existing users do?
There are two constraints: no duplicate usernames (obviously) and no duplicate addresses. (A system setting I believe)
What I think I want to happen:
Look at the email. If it is a duplicate, then do nothing (except if there is a request to add user to a course, to do so) ie we have assumed there is a user there, and they have a unique address, and just keep them on the books. But don't create another account, don't change the username.
Look next at usernames. If it is a duplicate username, but a unique address not already in the system, then it is to create a username+digit and proceed.
And to flag it to me. This means there MAY be a problem with one person with two accounts and different emails. I can live with this.
I now have a situation where Iget everything. Duplicate usernames, duplicate emails, etc etc.