Many years back, when my creative juices started flowing, I came up with something which, I thought, was truly original, and only after I put the finishing touches on it did a product hit the market which boasted my exact effect! I bought the trick, and as it turned out, both my and this other fellow's handlings were equally stupid. Ah well, c'est la vie.
As it was pointed out earlier in this thread, if you do find that your creation is a reinvention of something old, most of the time, you can take solace in the fact that you're thinking is along the same lines as, perhaps, one of the greats of magic.
Also, say you were to find something you came up was a reinvention, and you wanted to perform it. Obviously, laymen don't care whether the trick you're doing was invented by Tarzan or Frankenstein, they're not there performing! Reinvention only seems to matter when dealing with the publishing of material.
An old magician friend of mine used to say:
"It's amazing what we can accomplish when no one cares who gets the credit."