Having already warned above (in Pasuk 15) against perversion of justice, why does the Pasuk repeat it here?
Rashi: The Torah here is issuing a warning to every individual who does business not to weigh and measure dishonestly, since he has the status of a judge, and a judge who perverts justice is described as 'hated, abominable, banned, and despicable;' he contaminates the land, desecrates Hashem, drives the Shechinah away, causes Yisrael to fall by the sword and exiles them from their land. 1
Oznayim la'Torah: Consequently, one should make a point of shopping by a shopkeeper who is scrupuously honest - Kal va'Chomer from Beis-Din, in connection with which Chazal said 'Halach achar Beis-Din Yafeh' despite the fact that the majority of Dayanim do not take bribes. See Oznayim la'Torah.
What is the definition of "Midah" of "Mishkal" and of "Mesurah"?
Rashi (citing Bava Metzi'a, 61b) and Rashbam: "Midah" refers to measuring land, 1 "Mishkal", to weighing food etc. 2 and "Mesurah", to measuring liquids. 3
Moshav Zekenim #1 (citing Bava Basra 89b): Mesurah is a tiny measure, one thirty-sixth of a Log (between nine and sixteen milliliters). 4
Moshav Zekenim #2: Mesurah is a rod (used to level off the contents in a container). 5
Bava Metzi'a 61b: "Mishkal" forbids placing one's weights in salt 6 . "Mesurah" forbids to be Marti'ach. 7
Targum Yonasan: A warning against using the same measuring-rope when measuring for two partners, for one partner in the summer, and for the other, in the winter.
Bava Metzi'a, (Ibid.): A warning against dipping one's weights in salt - Refer to Answer #4, note 1.
Targum Onkelos and Targum Yonasan translate "Mesurah" as volume - with regard to the different customs of selling, heaped or level. See Bava Basra, 89a. Refer also to answer #4, note 2.
One Log = 6 egg-volumes
The Gemara in Bava Basra 89b rules that it should be made of the wood of an olive or nut-tree. R'mah: Because if it is made of metal, it sinks into the Kli, and the measure will not be full. Re'em, in Shitah Mekubetzes: The measure will remain heaped, and the seller loses.
Riva: Rashi explains that this makes the weight heavier and the Torah is warning the purchaser. R. Tam citing the Yerushalmi, explains that it makes the weight lighter, and the Torah is warning the seller.
Rashi (Ibid.): This entails pouring the liquid quickly causing it to foam and consequently, to shortfall the customer. The Shitah Mekubetzes however explains that it entails heating the liquid, which (temporarily) increases the volume. See also Torah Temimah, note 264.
