check for duplicate data

Greeting from Malaysia hi, i am a beginner in VB.NET. How do I check the duplicate records in the database? For example, I have a table called category in SQL server. This table contains with CategoryID and Category. In the front end I created a form that contains only 2 textbox. One is for the category ID that the ID is auto generated and the other textbox is for the user to enter the categoris(ed. adult, kids, comedy,.......) . So, how do I check if the user enters the category that already exists in the table. Please help! Have a nice day! Thanks and regards, Vivian