Both in legal and ethical terms do you think this is a good thing to do?
Legal? Well... it's all about (international) patents. Is the business of the startup patented? Or even patentable? Seem not realistic (but some folks patented the "one-click-buy button")... Will you copy the tools or methods (that are more easily patentable... but need to be patented "worldwide"... ;) )? Or can you try something more "french" ?
All in all, your problem will be to move faster and be better for your market than teh startup you're copying. Because they will come if they succeed, sooner or later. Good luck :)