Basically, say there are 500 engineering requirements in a database. If 2/5 of these requirements are duplicates, I'd like to calculate the probability that if a person started reading requirements at random, without repeating any, what is the probability after reviewing X requirements that at least on pair of duplicates were read.

As an example, say I start going through the requirements and after reading X, what is the probability I have pulled and read any redundant pair?

As an extended generalization, what would the equation be if I knew that 2/5 were redundant and of those 1/5 were triplicates? Here I'm interested in the probability of viewing at least two of the same requirement.

Thanks!