PHP Registration Page Security Check / Fix

Hello, I have website that can register account and the database i use is mssql. Now on registration page there is a check that it will check weather that Login name already exist on the database or not. if its already exist then it will give them error of course now the problem is a hacker can manage to bypass that check and able to register existing login name on database that already exist so by doing that he can login on someones account using his password he made on the registration page. now what i need is for someone who can investigate my registration page or any of my whole web scripts because i dont know how the hacker can manage to bypass the check maybe exploit on get post i dont know.. now if you ask my why dont put primary key on login name well i cant because on 1 table im only allowed to put 1 primary key and i have to put that on GUID its the identifier of someones account. So please help me with this problem on my web. ## Deliverables 1) Complete and fully-functional working program(s) in executable form as well as complete source code of all work done. 2) Deliverables must be in ready-to-run condition, as follows? (depending on the nature? of the deliverables): a)? For web sites or? other server-side deliverables intended to only ever exist in one place in the Buyer's environment--Deliverables must be installed by the Seller in ready-to-run condition in the Buyer's environment. b) For all others including desktop software or software the buyer intends to distribute: A software? installation package that will install the software in ready-to-run condition on the platform(s) specified in this bid request. 3) All deliverables will be considered "work made for hire" under U.S. Copyright law. Buyer will receive exclusive and complete copyrights to all work purchased. (No GPL, GNU, 3rd party components, etc. unless all copyright ramifications are explained AND AGREED TO by the buyer on the site per the coder's Seller Legal Agreement). ## Platform Windows Server 2003 Enterprise Edition R2 SP1 Xampp Apache Webserver PHP 5