Kommentar zu Wajikra 25:43

לא תרדה בו בפרך THOU SHALT NOT RULE OVER HIM WITH RIGOUR — i. e. do not force him to do some useless work with the sole purpose of vexing him. Do not e.g. say to him, “Warm me this cup of drink" when this is unnecessary; nor: “Hoe beneath this vine tree until I come" whilst he does not know when you may be expected to come back. Perhaps you will say, “No one knows the fact — whether it is necessary or not, so I will tell him that it is necessary”! — the thing is thus entrusted to his (the master’s) heart only, therefore Scripture states, “but be afraid of thy God!” (Sifra, Behar, Section 6 2; see Rashi on Leviticus 19:32).

You must not rule over him to crush him. Giving him work he is not capable of performing has no use for the master and is only to crush him so that he will not consider himself a free man, and this is prohibited with a Hebrew servant.

לא תרדה בו, “do not force him to perform unnecessary tasks just to keep him busy.” The reason why the Torah repeats this warning is because, sadly, exploitation of slaves has become a common practice.