Using Xaml and get "Additional information: An item with the same key has already been added."

I have the following Xaml:

<ContentPage.Content>
    <ScrollView>
      <ListView ItemsSource="{Binding GroupedItems}" IsGroupingEnabled="true" GroupDisplayBinding="{ Binding Name }" GroupShortNameBinding="{ Binding ShortName }" IsEnabled="False">
        <ListView.ItemTemplate>
          <DataTemplate>
            <TextCell  Text="{Binding Title}" Detail="{Binding Text}" IsEnabled="False"></TextCell>
            <Grid HorizontalOptions="CenterAndExpand">
              <Image></Image>
              <Image></Image>
              <Image></Image>
              <Image></Image>
              <Image></Image>
            </Grid>
          </DataTemplate>
        </ListView.ItemTemplate>
      </ListView>
    </ScrollView>
  </ContentPage.Content>

I keep getting the following exception:

_An exception of type 'System.ArgumentException' occurred in mscorlib.ni.dll but was not handled in user code

Additional information: An item with the same key has already been added._

What is it referring to? I have had this on a number of occasions but not sure what rules I'm breaking...