Even though Moodle spots that it is a duplicate email address, an email that differs by case in the CSV file will prevent the user details being updated (as opposed to one that does not).

To reproduce. Three example (tab delimited) files are attached

• use Upload Users with default settings and file test1.txt to create a new user - Fred Bloggs
• now use test2.txt, selecting 'Update existing users only' and 'Existing user details' = 'Override with file and defaults'.
• user is updated, location is changed to Manchester
• now repeat the above using test3.txt
• Update fails.

The only difference in test3.txt is that the email is changed in case. Moodle still indicates that it is a duplicate email (as it does for test2.txt) but it seems to be enough to break the process.