The crew may have entered it on the wrong page, and the database entry only may have been done for both coupons in SYD.
Thought about that scenario. The coupons are entered into a database by an Engineer directly from what is written on the log coupons.
Prior to a flight if there was no defect, the crew must write "Nil" or "Nil Defects". So if this was the case and done properly, why would the crew accidently enter a defect into an already used coupon?