多出来的is是谁给抄错了.原文是一套测验题,正确的句子是：
This it certainly is,but this is by no means its only achievement,for there are many suggestions for further lines of research,and the discursive passages are crisply provocative of new ideas and new ways of looking at established findings.
所以你第二个问题不存在,只有一个is.要回答第一个问题,需要看这句前面的一句.我给你加上：
As a result,the book seems at first sight to be an exhaustive and critically useful review of the literature.This it certainly is,but this is by no means its only achievement,for there are many suggestions for further lines of research,and the discursive passages are crisply provocative of new ideas and new ways of looking at established findings.
这样看,This it certainly is中的This是指前一句的表语,而it指的是the book.这是个倒装句,翻过来就是it certainly is this.把两个代词用它们所代指的替换,就是The book certainly is an exhaustive and critically useful review of the literature.也就是说,这句话把前一句基本上重复了一遍,只不过改变了强调并且更加肯定.倒装的原因是为了使this更清楚地指向前一句的表语.