Ahh, good, now we're getting into specifics.
In your model organism of choice, name a specific gene associated with leg development that, when duplicated or up/down regulated, results in a "freak." I'm assuming you must know one, or have read about one, to have such a strong opinion on the subject.
This way, we'll hopefully get a clearer picture of what a "freak" is.
Secondly, my original question still applies. Independent of gene product (though specifically in the family of genes associated with leg development, but you will need to clearly specify which genes are in this family), why is an arbitrary gene duplication event deleterious to the individual?
Simply show an organism where a gene duplication or regulatory mutation did not produce a 'freak'. We can discuss fitness 'advantages' from there.
Should be relatively simple, if evolution is true.