Yes, and you can put a number on the probability of that happening, say a fixed p << 1. And then you can choose the number of required matches before flagging,

Yes, and you can put a number on the probability of that happening, say a fixed p << 1. And then you can choose the number of required matches before flagging, say N. And then (assuming independence [1]) you have an overall probability of p^N, which you can make arbitrarily small by making N sufficiently large. (I'm pretty sure that's how Apple came up with their "1 in a trillion chance per year".) And then you still have manual review. [1] you could "help" independence by requiring a certain distance between images you simultaneously flag.