Check if there is a duplicate username in MySQL database

Good day! I am new in vb.net and I want to detect if there is a duplicate of the record that a user will try to add. If the user adds an existing username from the database then it will show an error that the username has already been used.
Here is my code:
conn.Open()

        Dim qadd As String = "INSERT INTO tbl_user (uname, pword, ulvl) VALUES ('" & txt_uname.Text & "' , '" & txt_pword1.Text & "' , '" & txt_pword2.Text & "') ON DUPLICATE KEY UPDATE uname = ' " & txt_uname.Text & " '"
        Dim cmd As New MySqlCommand(qadd, conn)
        Dim data As MySqlDataReader = cmd.ExecuteReader
        MsgBox("User has been added!", MsgBoxStyle.Information)
        txt_uname.Clear()
        txt_pword1.Clear()
        txt_pword2.Clear()
        txt_uname.Focus()
        conn.Close()