This has been discussed a lot.
To put it simply, they look similar because they both set out to achieve the same goal, which was to be the fastest car possible, the only difference being that the Caparo had a few restricions.
There's most likely no copying involved, it just the fastest shape aerodynamically. It's the same reason that all F1 cars look nearly identicle, not because it's compulsory for then to look like that, but because it's the fastest shape possible whilst keeping within the restrictions.