To teach is to learn twice.

[Enseigner, c’est apprendre deux fois.]

Joseph Joubert (1754-1824) French moralist, philosopher, essayist, poet

Pensées [Thoughts], ch. 19 “De l’Éducation [On Education],” ¶ 88 (1850 ed.) [tr. Collins (1928), ch. 18]
    (Source)

(Source (French)). Alternate translation:

To teach is to learn twice over.
[tr. Lyttelton (1899), ch. 18, ¶ 18]