Statistics can be weird. Try writing down the outcome of an imaginary coin toss 100 times as you imagine it, then try tossing an actual coin 100 times, and see how many runs of heads and tails you get which are longer than 8, 9 or even 10. Random can often look much less random than the pseudo random humans create.
I've read a case on this subject before and IIRC what is supposed to be random is the order in which employees are tested, and the timing, not the identity of the testee in any given test week. So it's possible that you might be tested in week 1 of the first 40 week cycle, and also in week 1 of the second 40 week cycle. It's random you got the first week both times. But twice in a period of 7 days is not, I think, justifiable. I think it's inconsistent with an employers obligation for fair dealing, and could be a breach of the implied term of trust and confidence, to subject an employee to repeat testing in that way
If there is no explicit term in his contract (or a term in the health/safety or disciplinary policy that meets the bar for incorporation as a term of his contract), then he may well have borne more than is required simply by acceding to the first test given he doesn't work with heavy machinery. And I think the bar for incorporation may well be a high one; if he doesn't work with heavy machinery etc, then an employer who seeks to make random testing a part of their workplace practice really should explicitly identify it before offering the job and explicitly include it as a condition of employment
