We mentioned sentences need to be concise and tight. We must avoid needless words. The use of "who" and "which" sometimes result in such redundancy.
Before: He is a man who is very sly.
After: He is very sly.
Before: Disneyland is always a theme park which I have always wanted to visit.
After: I have always wanted to visit Disneyland.
Before: His mother, who was a former nurse, helped the patients.
After: His mother, a former nurse, helped the patients.
Before: Music, which is his first love, influenced his lifestyle.
After: Music, his first love, influenced his lifestyle.