After renaming user the Sprint Health Gadget displays a ghost user

Steps to reproduce

1. Create a sprint and add some assigned issues to this sprint
2. Add the "Agile Sprint Health Gadget" in your dashboard
3. Check the option to show the assignees
4. Verify that the user appears in the gadget
5. Browse through the Users in JIRA's Administration and rename one of the assignees
6. Browse back to the gadget and verify that there is a "ghost user" with the original name, and the actual user with the modified name

Current behaviour

The gadget is duplicating the user, keeping a ghost user with the original name and the actual user with the modified name

Expected behaviour

Remove the ghost user and keep only the actual user