Inserting by multiple forms.

I have a form used my multiple users. The form has to create a proprietary identifing number. I check to see what the last number is, then increment it and then check to make sure the number i currently have is not already in the database. This increment-and-check logic is in a loop until i find a number not in the database. I am still having trouble though, when multiple users click on the submit button at the same time. What can I do to solve this issue? I am using PHP for the webpage and have an MSSQL database.