Why does the Torah write "Es Kutonto, Es Kesones ha'Pasim Asher Alav"? This seems repetitive.
Rashi: Because they stripped him of the two robes that he was wearing, the one that he wore regularly and the one that his father made specially for him. 1
Mizrachi: He was wearing only one robe - and "Es Kesones ha'Pasim Asher Alav" is coming to explain "Es Kutonto." Ha'amek Davar - You cannot say that they cast him in naked, and he was sold naked!
Oznayim la'Torah: Yosef was wearing two robes, and they stripped him only of the Kesones ha'Pasim. See his proof.
Moshav Zekenim, Hadar Zekenim, Riva: Presumably, the special one was on top, in which case, they should have removed it first! They removed from the bottom (the regular robe was longer). Alternatively, travelers normally put a [simple] robe over their [dignified] clothes to cover them - Moshav Zekenim. Ohr ha'Chayim - they intended only to remove his special robe on top, which they envied. In their fury, they were not careful to remove only it, and they clutched also his inner robe with it, and threw him into the pit naked.
Why did they strip him of his special robe?
Rashbam: In order to dip it in goat's blood, to mislead their father into believing that a wild beast had killed him. 1
Ohr ha'Chayim, Malbim: It was this one that they envied. Ha'amek Davar - Also, they wanted to bring it to their father (37:32). And even if it were the same as their robes, when a distinguished person sins and must be punished, first we remove his distinguished robe (a sign of his importance), and then we lash him (Yerushalmi Horayos 3:1).
Rashbam: Not because it was the cause of their hatred. Refer also to 37:23:1:1*.
QUESTIONS ON RASHI
Rashi writes: "'The Kesones ha'Pasim' - which his father had added." What does Rashi mean?
Gur Aryeh: The repeated terms, "his Kutones; the Kesones Pasim," indicate that he wore two tunics. Does one wear a doubled garment, one on top of the other? Yaakov had given Yosef an additional garment made of Pasim, to indicate his stature. 1 Yosef didn't wear it against his body so that it would not become soiled with sweat; rather, he wore it over his Chaluk (robe). Our verse means, "they stripped him [both] of his robe, [as well as] the Kesones ha'Pasim that was over [his robe]."
Gur Aryeh interprets the verse regarding the Kesones Pasim of Tamar (Shmuel II 13:18) in the same way.