secs_wpf\MainWindow.xaml.cs

using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace secs_wpf
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void ButtonAddName_Click(object sender, RoutedEventArgs e)
        {

            if (!string.IsNullOrWhiteSpace(txtName.Text) && !lstNames.Items.Contains(txtName.Text))
            {
                lstNames.Items.Add(txtName.Text);
                txtName.Clear();
            }
        }
    }
}

secs_wpf\MainWindow.xaml

<Window x:Class="secs_wpf.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:secs_wpf"
        mc:Ignorable="d"
        Title="SecsTest" Height="450" Width="800">
    <Grid Margin="10">

        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>

        <Label Grid.Column="0" Grid.Row="0">Names</Label>
        <!-- lstNames.Items.Add(txtName.Text); -->
        <ListBox Grid.Column="0"  Grid.Row="1" x:Name="lstNames" />


        <StackPanel Grid.Row="1" Grid.Column="1" Margin="5,0,0,0">
            <TextBox x:Name="txtName" />
            <!-- private void ButtonAddName_Click(object sender, RoutedEventArgs e) -->
            <Button x:Name="btnAdd" Margin="0,5,0,0" Click="ButtonAddName_Click">Add Name</Button>
        </StackPanel>


    </Grid>

</Window>