I do think self-help can be toxic, but I don't believe it's because of its repetition. There is repetition in philosophy, religion, cooking, yoga, etc. Unless it's literature--in which narrative arcs are often repeated, too--it's not original. The problem with self-help is identified in your article: it preys on the desperate and puts the entire onus on the individual to change. This isn't always bad because people seeking self-help want to change but the advice is often far too broad. That's why there is an entire self-help series called For Dummies and another called For Idiots. Both series are more useful than standard self-help because they are more specific.