Q:

The property content is set more than once.. WPF

so i had the same problem as this person.. WPF: Bind Collection with Collection to a ListBox with groups
I tried to implement the suggested answer to my problem but found the error "The property 'Content' is set more than once".  The only difference is id like to implement this in a User Control like so:
here is my c# code:
public class Dog
{
    public int dogID { get; set; }
    public String Name { get; set; }

    List<Puppy> puppies { get; set; }
}

public class Puppy
{
    public String Name { get; set; }
}

and this is XAML:
<UserControl x:Class="PetStore. Menu"
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
            xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
            mc:Ignorable="d" 
            d:DesignHeight="300" d:DesignWidth="300">

    <CollectionViewSource x:Name="dogs" Source="{Binding}" >
        <CollectionViewSource. GroupDescriptions>
            <PropertyGroupDescription PropertyName="Name" />
        </CollectionViewSource. GroupDescriptions>
    </CollectionViewSource>

    <DataTemplate x:Key="dogTemplate" DataType="Project">
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="{Binding Name}" />
        </StackPanel>
    </DataTemplate>

    <ListBox ItemsSource="{Binding Source={StaticResource dogs}}">
        <ListBox. GroupStyle>
            <GroupStyle HeaderTemplate="{StaticResource dogTemplate}" />
        </ListBox. GroupStyle>

        <ListBox. ItemTemplate>
            <DataTemplate DataType="Puppy">
                <ListBox ItemsSource="{Binding puppies}">
                    <ListBox. ItemTemplate>
                        <DataTemplate DataType="Puppy">
                            <TextBlock Text="{Binding Name}" />
                        </DataTemplate>
                    </ListBox. ItemTemplate>
                </ListBox>
            </DataTemplate>
        </ListBox. ItemTemplate>
    </ListBox>
</UserControl> 

Any help is greatly appreciated!! 

A:

You're missing the UserControl. Resources tag:
<UserControl x:Class="Tackle. View. Menu"
             ...>

    <UserControl. Resources> <!--  You're missing this -->

        <CollectionViewSource x:Key="dogs" Source="{Binding}" > <!--  Change x:Name to x:Key here -->
            <CollectionViewSource. GroupDescriptions>
                <PropertyGroupDescription PropertyName="Name" />
            </CollectionViewSource. GroupDescriptions>
        </CollectionViewSource>

        <DataTemplate x:Key="dogTemplate" DataType="Project">
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="{Binding Name}" />
            </StackPanel>
        </DataTemplate>
    </UserControl. Resources>

   <!--  here goes your window content -->

The error message tells you what you did wrong: you attempted to set the Content property more than once.   When you declare an element as a direct child of a control, then you are implicitly setting whichever property is specified in the [ContentProperty("...")] attribute on that control type.   For ContentControl (and its subclass UserControl), that property is Content.   If you want to set a different property, or add something to a collection/dictionary property, you must specify the property name.   This means either using the Property="value" attribute syntax or the <Owner. Property> element syntax.