Prevent duplicate data in visual basic

Private Sub insert_category()
        Call connection_string()
        MysqlConn.Open()
        Query = "SELECT * FROM tbl_categories WHERE CategoryName = @CategoryName"
        COMMAND = New MySqlCommand(Query, MysqlConn)
        COMMAND.Parameters.AddWithValue("@CategoryName", txtCategoryName.Text)
        READER = COMMAND.ExecuteReader
        If READER.HasRows Then
            'category already exists
            MsgBox(txtCategoryName.Text & " " & "already exist.Enter another name and try again", MsgBoxStyle.Exclamation, "Product")
            MysqlConn.Close()
        Else
            Try
                Call connection_string()
                MysqlConn.Open()
                Query = "INSERT INTO tbl_categories (CategoryName) VALUES (@CategoryCode,@CategoryName,@Datecreated,@status)"
                COMMAND = New MySqlCommand(Query, MysqlConn)
                COMMAND.Parameters.AddWithValue("@CategoryName", txtCategoryName.Text)
                READER = COMMAND.ExecuteReader
                MysqlConn.Close()
                MsgBox(txtCategoryName.Text & " " & "added successfully!", MsgBoxStyle.Information, "Success")
                txtCategoryName.Text = ""
                txtCategoryName.Focus()
            Catch ex As Exception
                MsgBox(ex.Message)
            End Try
        End If
    End Sub