Double Record In DB

Dear All
I have a code to insert OperatorName and Password to the database.
But I have a concern that if operator records his/her name twice to the DB then it will be double records which I do not want this to happen in my code.
How can I write a vb.net Code to check UserName in the Database before save it.
Many Thanks for any help
salih

In your database define the filed to be unique and in your code catch the System.Data.ConstraintException