Updated insert method to allow items to be added to the end of the list.

C#/Datastructures/DoublyLinkedList.cs

@@ -105,9 +105,12 @@ namespace C_.Datastructures
             }
 
             //Set tail to new item if index is the end
-            if (index == Count)
+            if (index == Count - 1)
             {
+                //Decrement count as it will be be re-incremented once appended
+                Count--;
                 Append(value);
+                return;
             }
 
             //Fetch point in list and add new item