Fix CreateLookup throwing on same lookup creation

src/gaemstone.ECS/Entity.cs

@@ -50,7 +50,8 @@ public unsafe readonly partial struct Entity<TContext>
 	public Entity<TContext> CreateLookup<T>()
 	{
 		ref var lookup = ref Lookup<TContext>.Entity<T>.Value;
-		if (lookup.IsSome) throw new InvalidOperationException(
+		if (lookup == this) { /* Don't throw if lookup already has the same entity set. */ }
+		else if (lookup.IsSome) throw new InvalidOperationException(
 			$"The lookup for type {typeof(T)} in context {typeof(TContext)} is already in use by {lookup}");
 		lookup = this;
 		return this;