If you are asking about us, the Readers of your question, I can accept the existence of other 'me's around, as long as we all are more or less happy!
Nevertheless we should never meet, not even in pairs, the risk of a painful death by boredom being too high.
Regarding yourself my educated guess, from the lack of information, is that you don't have an identical (monozygotic) twin.
This is just a demonstration of what Jura wisely anticipated about our answers.
I can't refrain from a pedantic note: of course you mean 'substantially unique', otherwise also each ant would be unique in the whole Universe.