Is it a ban for one mistake?

My client accidentally copied his website and linked to the copied page from the main page. Then overnight the main keyword dropped from position 1 to position 80, and it hasn't recovered since then. (Many other keywords also dropped down.) It was a month ago. We have already solved the problem. The search console does not indicate a penalty.
Do you have any idea what could we do?

Can I ask, how did your client dupe the website? Did they create a Dev site and forget to block all robots and thus this site was crawled and indexed with a fully dupe site or did he point another domain or subdomain to the same web server?

myk33 said:

Can I ask, how did your client dupe the website? Did they create a Dev site and forget to block all robots and thus this site was crawled and indexed with a fully dupe site or did he point another domain or subdomain to the same web server?

Click to expand...

I don't know how he did, but he wanted to redirect one of his smaller website (with different topic) to his main, bigger website, but he accidentally copied the content of the main website to the little site.
The main site sell dietary supplements and potency engancer, the little site sold cbd oils.

AND what is the best... he add links from the potency site's main page to the cbd oil site (with full of copied content).

I don't know how this happened. The Google indexed the copied content page.

But it was a month ago. We solved the problem but the positions were not restored.

(Sorry for the dummy language skills.)