Details
Nano ID is quite comparable to UUID v4 (random-based). It has a similar number of random bits in the ID (126 in Nano ID and 122 in UUID), so it has a similar collision probability.
The main difference from UUID v4 is Nano ID uses a bigger alphabet, so a similar number of random bits are packed in just 21 symbols instead of 36.
While the probability of generating the same Nano ID is non-zero, for most practical purposes, you can assume it is unique and non-repeatable.