1. Browse to users
2. Click to edit a user in the ajax window
3. Change user host
4. Pick delete the old one from the user tables.
5. Save changes
6. See the user double in the user table. Once with old and once with new host.
Refreshing the user list shows only the new version so it's only an ajax / display problem.
Version: 3.5.3
Thanks for reporting. Fixed in repository