The field in the Assign Issue dialog allows for either typing to immediately select somebody, or clicking on the down arrow to scroll through a list of all the users on an instance. If you have enough users on your instance that the dropdown list has a scrollbar, if you scroll to the bottom JIRA appears to query itself for the list of assignable users multiple times, and you end up with duplicate users appearing at the bottom of the list.
- Create 15-20 users in JIRA 6.5-OD-07, ensure all of them are able to be assigned issues (you need enough users to cause the user list in the Assign Issue dialog to have to scroll).
- Create a new JIRA issue.
- Click Assign, then hit the dropdown arrow in the Assignee field.
- Scroll down (using the keyboard or trackpad/mouse, it doesn't matter) to the bottom of the list.
You reach the bottom of the user list and there's no duplicates.
As you get to the bottom of the user list, you'll notice the scrollbar will jump and you'll have a couple of duplicate users appearing at the bottom. This will repeat two or three more times.
You can see the XHRs appearing in the web inspector each time the scrollbar jump happens. The very first XHR when you click the down arrow in the Assignee field will contain all the users with no duplicates, but the subsequent ones will have only one or two users in them, which are the duplicates that appear at the bottom of the list upon scrolling down.
This isn't browser-specific (I can replicate it in Firefox, Chrome, and Safari).
Currently there's no workaround, but this issue is purely cosmetic.