In fact, 99.999999% is an extremely low estimate. The number of ways that a deck of cards can be shuffled is 52! Which is equal to 8065817517094387857166063685640376697528950544088327782400000000000 possibilities.
If you shuffled cards every second from the birth of the universe until now, you still wouldn’t even come close (statistically) to getting the same arrangement twice.
What’s fun is how often this principle is used every day. For example, when you upload a video to YouTube, you’re assigned a unique URL, but it would be too slow to simply add your URL to a list to make sure nobody else uses it. There are millions of videos uploaded every day, and thousands of servers spread all over the world.
Instead, YouTube just generates a truly random URL and depends on the odds of two videos having the same URL being effectively zero.
The same is true for Bitcoin. If you could guess a Bitcoin private key for any currently used wallet, you’d have full access to the funds within that wallet. This can even be done offline. Even if you could guess trillions of private keys per second, the odds of you hitting even one that’s already been used is low enough to be totally secure.
For generating unique IDs, you can combine a random part with some other data, like the current time, so if it’s only those that are generated within the same millisecond that you even need to worry about.
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