creating form in asp.net and error in SQL server

Question

hi all..

i have create a registration form in asp.net & SQL server..
everything was working good..
now the problem is when two user register at same time it'll generate same user ID for both..
the form will generate user ID when user click "Register" button. Until they fill up form and click "submit" it'll not save into database.

how can i avoid this problem..?what is the correct method to handle this problem..