It's like any thing else . . . two projects covering the same material or with the sam goal are liable to arrive at similar results. Take a look at some military aircraft, for example. Their are many superficial similarities between the Ki-61 and BF-109, or the B-52 and Tu-95, or the F-15 and F/A-18 and the MiG-29 and Su-27. However, these similarities are the result of designers building planes with to fill the same combat roles, not necessarily because they directly copied designs from one another.