Daf Yomi - Yoma 62 - No Two Snowflakes....

It is learned in the Mishna that both goats should have the same appearance.

Ritva says that even though it is impossible for two animals to be precisely the same, like it is stated in Yerushalmi that even two grains of wheat will not be exactly alike, nevertheless, they should try for them to be as similar as possible. Tosfos Yeshonim says that two people cannot appear the same as the Gemora in Sanhedrin states, however animals could be similar. Rashi in Sukkah says that even apples on a tree cannot be the same as another.