South Africa Law
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This would not be a copyright issue as copyright only attaches to things like artwork, literary works and music. This could be a patent infringement (unlikely) or an infringement on a registered design. In order for them to have rights that they can enforce (as they claim) they would need to show you that the design is registered with the CIPC and that they have exclusive rights on that particular design. Likewise if this is an infringement of a patent (which, again, I think is unlikely. The product does not seem capable of being patented).
Ask them to give you that before you will consider their request and if they can provide something to that effect, THEN go see an intellectual property attorney for more specialized and specific advice.
Interesting question. Let's assume it is art and they have copyright in it, are you infringing on it by producing other windmills? Let's take it to the absurd, as a lawyer friend of mine always says. If they can do that, it would mean that someone else could produce a painting with a protea as the main theme and then no one else in the world would be allowed to produce another painting of a protea ever. Or if someone makes a statue of a dog, then no one else is allowed to make a statue of the world.
What I am trying to say is that, if it can be classified as art, the fact that you are also making windmills does not necessarily mean that you are infringing on their copyright. Unless yours looks exactly like theirs, I doubt that they are going to succeed with a copyright infringement claim.