We have the scenario where we have a group of users that want alerts
when they are assigned a task. We create the alert and it works fine,
except for a single user. This one alert becomes two when we look at
Site Settings >> User Alerts. When I delete one of the two, it
deletes both. When I add one back in, SharePoint creates two. When a
task is assigned to this user, he is getting two emails.

This will quickly get very bothersome and the tasks will start to be
ignored.