On the Agent Tab -> Update Agent page, you see duplicate agents listed.
This page works differently from others. The page looks at the scheduler and returns a list of all the machines that have a particular agent procedure (the update agent procedure) scheduled.
If a machine has had a update scheduled, but for some reason did not complete, and then had the update scheduled again, you will see two entries for that machine, because there are two schedules.
WORKAROUND / RESOLUTION
This can be corrected by simply clicking the "select all" box at the top of the agent list, and clicking "Cancel".
This will remove all the schedules and return just the list of the agents again.
If there are agents that need updating, you can re-schedule these.