Rashi on Deuteronomy
ואצוה את שפטיכם AND I COMMANDED YOUR JUDGES — I said to them: be deliberate in judgment: if a certain point of law comes before you once, twice, three times, do not say, "This point of law has already come before me several times”, but discuss it well on that occasion also (Sifrei Devarim 16:1).
Or HaChaim on Deuteronomy
בעת ההיא לאמוד, at that time, saying, etc. The word לאמור, as in many instances, indicates that the words to be communicated were not intended to be literally these words and immediate. In this instance Moses did not command the judges to say specific things; he informed them that an essential part of the judicial process was to listen to the arguments of a litigant only when the opposing side was present.
Rabbeinu Bahya
ואצוה את שפטיכם, “I commanded your judges.” This refers to the judges being cautioned that although they might feel that similar cases had been judged by them many times, each case has to be addressed as if it were the first of its kind.