When I change assignee why is the Old Assignee and New Assignee notified?

Owned by Andy Brook [Plugin People] (Unlicensed)
Mar 08, 2015

Because thats what JIRA does, its not a bug, see https://confluence.atlassian.com/display/JIRA/Creating+a+Notification+Scheme 

JIRA will send notification emails to both the previous assignee and the current assignee, whenever the assignee field changes.