You need to either Rollback the database to a state where the user does not exists, or you need to delete the user from the data base, or you need to create a random username every time and then use it.
Normally, this is something your DBAs would do. If your QA group has a QA member who does this, ask him/her. If your QA group lets each QA person do this on their own, then you should ask your QA team or manager how to do it (there is probably some process involved so you don't rollback other testers who need the current data).
"The last 10% of any software project will take 90% of the budgeted time. The first 90% will take the other 90%"
Wouldn't this be esier/more how the system will be used?
Use a random name generator:
And copy the function below into your script
For i = 1 to 10