fix(ios): an instance of ListView crash during layout

src/Uno.UI/Extensions/EnumerableExtensions.cs

@@ -34,5 +34,43 @@ namespace Uno.UI.Extensions
 
 			return output;
 		}
+
+		/// <summary>
+		/// ToDictionary that doesn't throw on duplicated key. The first value is kept per key.
+		/// </summary>
+		public static Dictionary<TKey, TElement> ToDictionaryKeepFirst<TSource, TKey, TElement>(
+			this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector,
+			Func<TSource, TElement> elementSelector
+		) where TKey : notnull
+		{
+			var result = new Dictionary<TKey, TElement>();
+
+			foreach (var item in source)
+			{
+				result.TryAdd(keySelector(item), elementSelector(item));
+			}
+
+			return result;
+		}
+
+		/// <summary>
+		/// ToDictionary that doesn't throw on duplicated key. The last value is kept per key.
+		/// </summary>
+		public static Dictionary<TKey, TElement> ToDictionaryKeepLast<TSource, TKey, TElement>(
+			this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector,
+			Func<TSource, TElement> elementSelector
+		) where TKey : notnull
+		{
+			var result = new Dictionary<TKey, TElement>();
+
+			foreach (var item in source)
+			{
+				result[keySelector(item)] = elementSelector(item);
+			}
+
+			return result;
+		}
 	}
 }

src/Uno.UI/UI/Xaml/Controls/ListViewBase/VirtualizingPanelLayout.iOS.cs

@@ -591,13 +591,13 @@ namespace Windows.UI.Xaml.Controls
 				// We are layouting after an INotifyCollectionChanged operation(s). Cache the previous element sizes, under their new index 
 				// paths, so we can reuse them in order not to have to lay out elements with different databound sizes with their static size.
 				oldItemSizes = _itemLayoutInfos.SelectMany(kvp => kvp.Value)
-					.ToDictionary(
+					.ToDictionaryKeepLast(
 						kvp => OffsetIndexForPendingChanges(kvp.Key, NativeListViewBase.ListViewItemElementKind),
 						kvp => (CGSize?)kvp.Value.Size
 					);
 				oldGroupHeaderSizes = _supplementaryLayoutInfos
 					.UnoGetValueOrDefault(NativeListViewBase.ListViewSectionHeaderElementKind)?
-					.ToDictionary(
+					.ToDictionaryKeepLast(
 						kvp => OffsetIndexForPendingChanges(kvp.Key, NativeListViewBase.ListViewSectionHeaderElementKind).Section,
 						kvp => (CGSize?)kvp.Value.Size
 					);