do not return duplicate values from spatial hash

src/Collision/SpatialHash2D.cs

@@ -19,6 +19,7 @@ namespace MoonWorks.Collision
 		public int MinY { get; private set; } = 0;
 		public int MaxY { get; private set; } = 0;
 
+		private Queue<HashSet<T>> hashSetPool = new Queue<HashSet<T>>();
 
 		public SpatialHash2D(int cellSize)
 		{
@@ -68,6 +69,8 @@ namespace MoonWorks.Collision
 		/// </summary>
 		public IEnumerable<(T, IHasAABB2D, Transform2D)> Retrieve(T id, IHasAABB2D shape, Transform2D transform2D)
 		{
+			var returned = AcquireHashSet();
+
 			var box = shape.TransformedAABB(transform2D);
 			var (minX, minY) = Hash(box.Min);
 			var (maxX, maxY) = Hash(box.Max);
@@ -85,10 +88,13 @@ namespace MoonWorks.Collision
 					if (hashDictionary.ContainsKey(key))
 					{
 						foreach (var t in hashDictionary[key])
+						{
+							if (!returned.Contains(t))
 							{
 								var (otherShape, otherTransform) = IDLookup[t];
 								if (!id.Equals(t) && AABB2D.TestOverlap(box, otherShape.TransformedAABB(otherTransform)))
 								{
+									returned.Add(t);
 									yield return (t, otherShape, otherTransform);
 								}
 							}
@@ -97,6 +103,9 @@ namespace MoonWorks.Collision
 				}
 			}
 
+			FreeHashSet(returned);
+		}
+
 
 		/// <summary>
 		/// Retrieves objects based on a pre-transformed AABB.
@@ -105,6 +114,8 @@ namespace MoonWorks.Collision
 		/// <returns></returns>
 		public IEnumerable<(T, IHasAABB2D, Transform2D)> Retrieve(AABB2D aabb)
 		{
+			var returned = AcquireHashSet();
+
 			var (minX, minY) = Hash(aabb.Min);
 			var (maxX, maxY) = Hash(aabb.Max);
 
@@ -121,6 +132,8 @@ namespace MoonWorks.Collision
 					if (hashDictionary.ContainsKey(key))
 					{
 						foreach (var t in hashDictionary[key])
+						{
+							if (!returned.Contains(t))
 							{
 								var (otherShape, otherTransform) = IDLookup[t];
 								if (AABB2D.TestOverlap(aabb, otherShape.TransformedAABB(otherTransform)))
@@ -133,6 +146,9 @@ namespace MoonWorks.Collision
 				}
 			}
 
+			FreeHashSet(returned);
+		}
+
 		/// <summary>
 		/// Removes everything that has been inserted into the SpatialHash.
 		/// </summary>
@@ -150,5 +166,22 @@ namespace MoonWorks.Collision
 		{
 			return ((long) left << 32) | ((uint) right);
 		}
+
+		private HashSet<T> AcquireHashSet()
+		{
+			if (hashSetPool.Count == 0)
+			{
+				hashSetPool.Enqueue(new HashSet<T>());
+			}
+
+			var hashSet = hashSetPool.Dequeue();
+			hashSet.Clear();
+			return hashSet;
+		}
+
+		private void FreeHashSet(HashSet<T> hashSet)
+		{
+			hashSetPool.Enqueue(hashSet);
+		}
 	}
 }