Take two indentical objects - it doesn't really matter what they are - place them side-by-side. Looking at them most people would argue that neither could be unique: because to be unique means that there can only be one - and here we have two. But then a scientist enters the room dragging in a trolley on which he has placed a scanning electron microscope. He carefully examines both objects with his apparatus. After much measuring - accompanied by a little head scratching - he triumphantly declares, 'Aha! these objects are not exactly identical. Ergo... they are, indeed, unique!' And so it is with everything. Take two of anything and place them side by side; they might look the same: but they are never, ever, exactly the same. When it comes to almost, infinitely complex organisms: like people, the chances of finding two exactly the same are well below absolute zero.
And even if we do have doppelgangers out there, somewhere, in the infinite Cosmos; there would still be dissimilarities: not least because they are out there and we are here.