Fix SetSnapShot for duplicate items (nhibernate#2806)

Fixes nhibernate#2802

Author: maca88

src/NHibernate.Test/UtilityTest/SetSnapShotFixture.cs

@@ -49,6 +49,16 @@ public void TestInitialization()
 			Assert.That(sn.TryGetValue(null, out _), Is.True);
 		}
 
+		[Test]
+		public void TestDuplicates()
+		{
+			var list = new List<string> { "test1", "test1", "test2" };
+			var sn = new SetSnapShot<string>(list);
+			Assert.That(sn, Has.Count.EqualTo(2));
+			Assert.That(sn.TryGetValue("test1", out _), Is.True);
+			Assert.That(sn.TryGetValue("test2", out _), Is.True);
+		}
+
 		[Test]
 		public void TestCopyTo()
 		{

src/NHibernate/Collection/Generic/SetHelpers/SetSnapShot.cs

@@ -33,7 +33,7 @@ public SetSnapShot(IEnumerable<T> collection)
 				}
 				else
 				{
-					_values.Add(item, item);
+					_values[item] = item;
 				}
 			}
 		}
@@ -70,7 +70,7 @@ public void Add(T item)
 				return;
 			}
 
-			_values.Add(item, item);
+			_values[item] = item;
 		}
 
 		public void Clear()