Well in this instance, yeah Shark came first but when Dirt Devil went after it, it wasn't necessarily a copy - more that Shark likely just sold the mold to them, or it was just a "free use" design at the time.
You'd have to do some digging around at patents and stuff to figure out exactly what was going on and who the original designer was and what his stipulations were.
It's just like Dirt Devil's Ultra hand vac design got re-used for many different companies too.