The answer, of course, depends on whether or not the name "duck" is a trademark! If it is, then the closest that something can get, without permission of the owner of the trademark, is "duck-like."
``UNIX'' is a trademark of X/Open Company Ltd., and NetBSD has not been branded with that trademark by X/Open. Therefore, NetBSD is not UNIX, it is at best ``UNIX-like,'' or, for the more paranoid, ``UN*X-like.''