"Prior to a flight if there was no defect, the crew must write "Nil" or "Nil Defects"."
I think you mean post flight.
"So if this was the case and done properly, why would the crew accidentally enter a defect into an already used coupon? "
If you are saying that no engineer was called in ADL, then one would have to assume both coupons were entered on return to SYD. The crew would have had both pages. Would not be the first time someone opened up the the wrong page, or even skipped a page.
"but it's probably not too bright posting that on a public forum"
I agree, esp when tracking who views database records is so easily tracked these days. It might cost someone their job.