Creating Unique Usernames For Users Of A Database

I am tryin to create a database of users for my system,each user is given a username based on their own name(surname and first name), however if 2 users with the same or similar name enter their details they will be given the same username,for example john drew would return drewj as thier username, so would jack drew, when a new user is entering their details how do i check that the username generated for them does not already exist,i.e i need to compare the username generated for the new user with all usernames already held in the database,if the username does already exist i want to add a digit to the end of the username how is this done using asp?