Recently, after the acceptance of a paper, the journal said some sentences were exactly copied in my manuscript. I was shocked as I was the first author. Later, we found that during the preparation of the revised manuscript, one of the co-authors exactly copied two sentences from a reference. This brought fear in my mind if I had ever been a co-author in an unintentionally plagiarized paper. I got access to the Turnitin from a friend at a different institute and checked all my previous papers.
I realized that 8-10 years ago, a visiting student in my lab had significant portions of the published papers plagiarized. The data and analysis were correctly done in my supervision, but significant parts of introduction, methods and some places discussion were plagiarized. I had read and edited the drafts of the papers, but I didn't realize that he had copied text from other papers. Generally, most sources are cited. The student already earned a PhD based on the work.
At that time, no-one in our country had access to the plagiarism detection software from institutions.
What should I do now?