It should overwrite the previous one.
There is no "queue" - it might get "moved down", if the editor sorts by submission date, and reviews in tha order of that list (which is one of the options) slow it down, yes, but the alternative is that the password-protected area of the site can't be reviewed, and the site may get denied. Your choice.
Resubmissions of the same URL to the same category aren't "looked down on", since they overwrite a previous one and cause an editor no extra work. They are just usually counter-productive on everyone's part - in this special case, though it would be productive.
Either put the username and pass in the description (which will obviously be removed before the site is listed), or provide an email address in the description where an editor can write to you to get the username and password - the first might be quicker for a review, and the second might be mroe appropriate if you have more stringent security concerns. I have seen both done in various areas of the directory.
Hope this helps.