看到英英字典中一句解释

impurity：impurities are substances that are present in a small quantities in anther substance and make it dirty or of an unacceptable quality.

并列式定语从句中同主语的情况可否省略做代词代替主语的引导词？

比如

Mr. Smith, who has a good command of French and English but who doesn’t know Chinese, finds it difficult to study Chinese literature. 

Mr. Smith, who has a good command of French and English but doesn’t know Chinese, finds it difficult to study Chinese literature. 

镜像的问题还有代词代宾语的等代指同一成分的。