גַּם־בְּנֵ֣י אָ֭דָם גַּם־בְּנֵי־אִ֑ישׁ יַ֝֗חַד עָשִׁ֥יר וְאֶבְיֽוֹן׃
Both low and high, Rich and poor together.
Rashi on Psalms
Both the sons of “adam” The sons of Abraham, who was called (Josh. 14:15): “the greatest man (האדם) among the giants”; the sons of Ishmael and the sons of Keturah.
Ibn Ezra on Psalms
"Also" - calling the sons of Adam who aren't wealthy, or who are and are well known. That is the reason for also having "sons of man", to include the rich and the poor.
Malbim on Psalms
"Also", all groups of people, respected and not, rich and poor, everybody is ignorant about this.