User login seems to be case sensitive with what the user typed in the CAS form. If a user typed his login with a capital letter by mistake, he's not found.

Example : john.doe creates his account as johndoe (since the "." char is not allowed in this plugin).

John now logs-in as john.Doe : another account johnDoe is created if xwiki.authentication.cas.create_user=1 and two different users are now listed.

If John logs-in with John.Doe, a third user JohnDoe will be created, etc...