It's because there is only one single place in the genome that you really want to track: the furin cleavage site (FCS). I assumed the wrong reason for using double CGG.
It's not to distinguish natural from lab-made viruses (although it does do that).
It's so that you can have a test in the lab for whether the FCS you have inserted is working or not. It's so that you can "check your work".
The unique spelling with double CGG is the only one out of the 36 possible configurations of arginine (the "R" in the "PRRA" FCS insertion) that allows you...
Richard Muller, co-author with Steven Quay of the WSJ article, states in his interview with Sky News Australia (Scientific report suggests Wuhan lab leak as origin of COVID-19, YouTube, 10 June 2021, at 5:40 mark) that CGG was the spelling of arginine "most used in the laboratory" in lab-inserted furin cleavage sites and was in fact used by Shi Zhengli at the WIV, as she reported in one of her published papers.
But Steven Quay's mammoth 193-page Bayesian Analysis of SARS-Cov-2 Origin (https://zenodo.org/record/4477081#.YMU0-S0ZNE4) puts the number at ...
The recent article by Steven Quay & Richard Muller in the Wall Street Journal attempts to bring the issue to a head by simplifying it down to two main points:
(1) The double CGG codons in the SARS2 furin cleavage site were deliberately designed by the 11 or 12 researchers who have created chimeric viruses as an unmistakable 'marker' for lab-made viruses so that you could always tell which future mutations evolved from a lab virus and which were naturally evolved. SARS2 has these tell-tale double CGG codons in its furin cleavage site, ergo it...
Re 1) the codons, according to Christian Drosten, have precedence for evolving naturally in viruses. That could be because viruses evolve much faster than e.g. animals. Source: search for 'codon' and use translate here: https://www.ndr.de/nachrichten/info/92-Coronavirus-Update-Woher-stammt-das-Virus,podcastcoronavirus322.html
The link also has a bunch of content about the evolution of furin cleavage sites, from a leading expert.
Given that full transparency from Chinese authorities is unlikely, assessing the probabilities is the best we can do. Fortunately, that has been done with with impressive scientific rigour by DRASTIC member Dr. Steven Quay MD, PhD in his technically detailed 193-page Bayesian analysis of 26 known facts about the outbreak:
https://zenodo.org/record/4477081#.YNAFry0ZNE4
which he explains in layman's terms in his interview with Julius Killerby (cited in my comment above).
The advantage of this approach is that it follows the scientific method: laying out clearly... (read more)