It happend to me as well. I almost fall from chair when i see that. Im doing SEO for some time, and this huge drop never happend to any of my websites.
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I will wait few days and see if it bounce back, and if not, i can make case study from it.
It was affiliate website with a lot of 301 redirects (expired domains) pointing to it, and partially generated / duplicated content.
I also lost snippets.
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URLS in index are the same.
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With this website i generaly try whats the limits ( because from my experience 301 redirects works very well ), and it work fine for this website as well ( website is live for 9 months +- ).
Lets see what will happend in next days.