Xaml propriété content definie plusieurs fois

Bonjour j'ai une question dans mon code xaml j'essaie d'ajouter certains controls pour manipuler ma datagrid sauf que quand j'ajoute les controles dans le code il dit la propriété 'Content' est définie plusieurs fois j'ai essayé d'ajouter des stackpanel et grid mais ça ne marche pas quelqu'un peut m'aider voilà mon code

<Window x:Class="demo.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:demo"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
        <StackPanel>
            
            <Border Background="#99FFFFFF" BorderBrush="#CCCCFF" BorderThickness="2" HorizontalAlignment="Left" Padding="20" Margin="20,10,0,0" Width="455">
                
                <TabControl Name="Tabs1" TabStripPlacement="Top" Background="LightGray" BorderBrush="Blue" Margin="9,10,4,5" Height="340">
                 
                    
                    <TabItem Header="Name" IsSelected="True" Background="Gray">
                        <DataGrid Name="dgEtudiant" VerticalAlignment="Top" HorizontalAlignment="Left" AutoGenerateColumns="False" RowBackground="Khaki" AlternatingRowBackground="Lavender">
                            <DataGrid.Columns>
                                <DataGridTextColumn Header="Matricule" Width="2*" />
                                <DataGridTextColumn Header="Prenom" Width="2*" />
                                <DataGridTextColumn Header="Nom" Width="2*"  />

                            </DataGrid.Columns>

                            <DataGrid.RowDetailsTemplate>

                                <DataTemplate>
                                    <StackPanel>
                                        <TextBlock Text="Adresse de courriel:"/>
                                        <TextBlock Text="james harden" Margin="10"/>
                                    </StackPanel>

                                </DataTemplate>
                            </DataGrid.RowDetailsTemplate>

                        </DataGrid>

                        <!--voila la ligne de l'erreur j'essaie d'ajouter des labels , button, puis textbox pour manipuler ma datagrid qui se trouve dans mon premier item -->
                        <Label HorizontalAlignment="Right" Margin="-30,42,186,22" Width="144" Grid.ColumnSpan="2" Grid.RowSpan="2">Nombre Joueur maximal:</Label>
                        <TextBox Name="Na" Grid.Column="1" Margin="19,41,-19,22" Grid.RowSpan="2" />
                        <Label HorizontalAlignment="Right" Margin="-30,42,186,22" Width="144" Grid.ColumnSpan="2" Grid.RowSpan="2">Nombre Joueur maximal:</Label>
                        <TextBox Name="Na" Grid.Column="1" Margin="19,41,-19,22" Grid.RowSpan="2" />
                        <Label HorizontalAlignment="Right" Margin="-30,42,186,22" Width="144" Grid.ColumnSpan="2" Grid.RowSpan="2">Nombre Joueur maximal:</Label>
                        <TextBox Name="Na" Grid.Column="1" Margin="19,41,-19,22" Grid.RowSpan="2" />

                        <Button> </Button>



                    </TabItem>
                    <StackPanel>
                  

                <TabItem Header="Image" Background="Gray">

                    </TabItem>
                    <TabItem Header="Hotels" Background="Gray">

                    </TabItem>
                    <TabItem Header="Contact Us" Background="Gray">
                        <Grid Height="150" Width="300">
                            <Grid.RowDefinitions>
                                <RowDefinition></RowDefinition>
                                <RowDefinition></RowDefinition>
                                <RowDefinition></RowDefinition>
                            </Grid.RowDefinitions>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="*"></ColumnDefinition>
                                <ColumnDefinition Width="2*"></ColumnDefinition>
                            </Grid.ColumnDefinitions>
                            <Label Margin="-25,-43,35,55">Ajouter un jeu</Label>
                            <Label HorizontalAlignment="Right" Margin="-30,0,186,14" Width="144" Grid.ColumnSpan="2">Nombre Joueur minimal:</Label>
                            <TextBox Name="Name" Grid.Column="1" Margin="19,-53,-19,66" />
                            <TextBox Name="Email" Grid.Column="1" Margin="19,0,-19,14" />
                            <Label HorizontalAlignment="Right" Margin="-30,42,186,22" Width="144" Grid.ColumnSpan="2" Grid.RowSpan="2">Nombre Joueur maximal:</Label>
                            <TextBox Name="Na" Grid.Column="1" Margin="19,41,-19,22" Grid.RowSpan="2" />
                            <Label HorizontalAlignment="Right" Margin="-30,33,186,31" Width="144" Grid.ColumnSpan="2" Grid.RowSpan="2" Grid.Row="1">Age minimum :</Label>
                            <TextBox Name="NbMinimum" Grid.Column="1" Margin="19,33,-19,30" Grid.RowSpan="2" Grid.Row="1" />
                            <Label HorizontalAlignment="Right" Margin="-30,67,186,-53" Width="144" Grid.ColumnSpan="2" Grid.Row="2">Numero de la catégorie:</Label>
                            <TextBox Name="AgeMinimum" Grid.Column="1" Margin="19,67,-19,-54" Grid.Row="2" />
                            <Label HorizontalAlignment="Right" Margin="-30,26,186,-12" Width="144" Grid.ColumnSpan="2" Grid.Row="2">Age maximum :</Label>
                            <TextBox Name="AgeMaximum" Grid.Column="1" Margin="19,25,-19,-12" Grid.Row="2" />


                        </Grid>
                    </TabItem>
                </TabControl>
            </Border>
        </StackPanel>
    </ScrollViewer>
</Window>