same userid multiple account

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previously I had replied to a similar thread here:

http://www.vbulletin.com/forum/showthread.php?367246-same-user-id-twice

I am told to create a new thread.
here is the problem:

i am on vb 3.8.4
I noticed that multiple user account can have identical username (it is identical username, it is not "1" replaced with "l" etc).

the problem is that we don't know how could this happen because when you try to create a new account with the same username at
domain.com/forum/register.php?do=register
if you type in the same username, then vb returns the message "That username is already in use or does not meet the administrator's standards. If you are 'tried username' and you have forgotten your password",
it would not allow you to create a username. So whoever created the same username, must created the same username not from register.php?do=register page.

What can be the problem? And what is the fix?

[Andy]

I suggest you restrict user names to a-z A-Z 0-9

Admin CP -> Settings -> Options -> User Registration Options -> Username Regular Expression -> ^[a-zA-Z0-9]+$

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Andy,
thanks for the regexp. I tried that don't know if it will fix it in the future because I have no way to test it. What do you suspect the problem? I don't think it is different character because I have matched those characters in the notepad.exe editor

[bulbasnore]

I've installed the regexp, now we'll see. FYI, charset collation is latin1_general_ci