Thereof, we find that the word (mankind) expresses three meanings in accordance with attributes of Allah (swt), the Lord, the Mighty and Majestic. This reflects the eloquence of the Arabic language. The whisperer could be of mankind or of Jinn. Thus, there is no repetition in this Surah.
يُوَسْوِسُ (whispers): Linguistically, in Arabic language, the whisper is a voice of attractive sweet talk and it indicates, as well, the sound of the gold jewelry, accordingly, it is said “the whisper of devil” because it has tinkle that attracts the hearing and attention. It is as if Allah (swt) is warning us from the devil and his seducing. So, if you went astray from Allah’s (swt) path the devil will dare whispers to you, whereas, if you remember Allah (swt) the devil retreats and weakens. The devil does not enter into battle with Allah (swt), rather, he does with Allah’s creations who forget Allah (swt) and go far from His method.
فِي صُدُورِ النَّاسِ (into the breasts of mankind): meaning into their hearts, souls, wills and their minds. It is not public as the whispering of people to each other.