Use a user name hook to normalize user names, or use a case sensitive 
authentication backend, or use a case insensitive collation for the 
rampage_users table.

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As you see below the user login as "jb.c@" and after he use "jb.C@" 
(uppercase C)
then we have an sql error and the error is reported in the web interface.

Logs:
Apr 24 10:14:10 webmail HORDE: [horde] Login success for 
jb.c@example.com [xx.178.164.94] to horde. [pid 13982 on line 163 of 
"/home/horde/login.php"]
Apr 24 10:14:11 webmail HORDE: [imp] Login success for 
jb.c@example.com (Horde user jb.c@example.com) [xx.178.164.94] to 
{bb.0.1.47:143 [imap]} [pid 9912 on line 183 of 
"/home/horde/imp/lib/Auth.php"]
..
Apr 24 11:13:11 webmail HORDE: [horde] User jb.c@example.com 
[yy.166.5.244] logged out of Horde [pid 10239 on line 118 of 
"/home/horde/login.php"]
...
Apr 24 15:19:04 webmail HORDE: [horde] Login success for 
jb.C@example.com [xx.178.164.94] to horde. [pid 13555 on line 163 of 
"/home/horde/login.php"]
Apr 24 15:19:05 webmail HORDE: [imp] Login success for 
jb.C@example.com (Horde user jb.C@example.com) [xx.178.164.94] to 
{bb.0.1.47:143 [imap]} [pid 22882 on line 183 of 
"/home/horde/imp/lib/Auth.php"]
Apr 24 15:19:16 webmail HORDE: [kronolith] SQL QUERY FAILED: 
SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 
'jb.C@example.com' for key 'rampage_users_user_name'  #012#011INSERT 
INTO `rampage_users` (user_name) VALUES ('jb.C@example.com') [pid 
13982 on line 815 of "/usr/share/pear/Horde/Db/Adapter/Base.php"]
...
Apr 24 16:09:26 webmail HORDE: [kronolith] SQL QUERY FAILED: 
SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 
'jb.C@example.com' for key 'rampage_users_user_name'  #012#011INSERT 
INTO `rampage_users` (user_name) VALUES ('jb.C@example.com') [pid 
22295 on line 815 of "/usr/share/pear/Horde/Db/Adapter/Base.php"]