How the the gemorah asks whats seems to be contradictions in two gemorahs and then answers that no each one is talking about the same case. For example in shabbos 89b on the bottom and 90a on top; it seems to me that its obvious??? what does the gemora want to teach us?
Yair Trebitsch, Brooklyn NY
In the Sugya you mentioned, the Gemara at first assumes that the Mishnah means to say that any spices can join together to make a Shiur. It asks from Chizkiyah, who says that only certain ones join together. The Gemara answers that the Mishnah is indeed referring only to those special spices.
Rashi (top of 90a) explains that the Gemara assumed the Mishnah must be discussing any spice, since it did not specify any special spices. Thus, the Gemara is asking if we should assume that there is a Machlokes about this Halachah, or should we accept a "forced" reading of our Mishnah (that reads unwritten words into the Mishnah) in order to avoid making a Machlokes Tana'im. The Gemara prefers the second possibility, and that is the Chidush of the Gemara.
I hope this helps you.