According to the NYTimes, high-end designers are fighting for copyright protection so knock-offs can't be produced the second a model walks down the runway wearing a new item. The copyright would last for three years. Retailers like H&M and Zara thrive on designer knock-offs. Other retailers play it safe and just use certain trends or attributes from high-end lines. This has been going on since the 1930's when American department stores would "borrow" designs from Paris haute-couture.
I highly doubt something like this would ever pass. And even knock-offs aren't exactly like the designs you see on a runway or in the window at Saks. So where would they draw the line?? There are so many variables in an item of clothing: color, fabric, cut etc. It would be impossible to lay out guidelines for what was considered a knock-off. If something like this would go through would it force retailers to be create and come up with their own styles??