C. F. Keil and Franz Delitzsch on Ecclesiastes 3:20

Ecc 3:20 “All goes hence to one place; all has sprung out of the dust, and all returns to the dust again.” The “one place” is (as at Ecc 6:6) the earth, the great graveyard which finally receives all the living when dead. The art. of the first העפר is that denoting species; the art. of the second is retrospective: to the dust whence he sprang (cf. Psa 104:29; Psa 146:4); otherwise, Gen 3:19 (cf. Job 34:15), “to dust shalt thou return,” shalt become dust again. From dust to dust (Sir. 40:11; 41:10) is true of every living corporeal thing. It is true there exists the possibility that with the spirit of the dying man it may be different from what it is with the spirit of the dying beast, but yet that is open to question.