You misunderstood the concept. ent_clone does not make a copy of the entity itself but of its mesh and skin. There is no entity removed and no new one created in your example. Instead you have just moved the original entity.

The proper usage is to first call ent_clone on an entity and then modify the mesh / skin as you see fits. The return value if ent_clone is the original entity, or NULL in case it failed.

So in summry:calling ent_clone let's you change an entities skin and mesh without changing the skin and mesh of the other entities sharing the corresponding model. ent_clone does not copy entities. You still have to create them with ent_create or place them via WED.

http://www.conitec.net/beta/ent_clone.htm