Samsung of all people has shown it's possible to make a thin notebook that's distinct:
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However most slim notebooks look like Macbooks because simply of the companies are afraid of failing and thus copy the successful design:
A good example is Dell. few years back, Dell made Adamo, which was actually a nice thin notebook that looked different from MacBooks.
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Unfortunately it was a failure and Dell eventually discontinued the model and replaced with an Ultrabook that looked like...
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In short, of course they are copying Apple's design cues and overall look&feel. There are plenty of other ways for the design but inevitably, successful design leaders get copied on their designs. However neither HP or Dell will admit anything nor do I expect them to.