It is evidently possible for there to be duplicate user entries in the userbase table:

jira-xdoclet=# select username from userbase group by username having count > 1;
username
------------
millettjon
(1 row)

jira-xdoclet=# select * from userbase where username ='millettjon';
id | username | password_hash
---------------------------------------------------------------------------------------------------------
12364 | millettjon |
12363 | millettjon | 9Ik/GVcirBUsMRbbMlDXfiVpeCtJL88sJThLI+Z01kGIZWlA61BuPgZv0MQ3pWRaHIHsnnwd23cCROFYAIrDhg==

This causes the User browser to show:

Displaying users 0 to of .

and not display any users.