What is our relationship to a continued dispute?
A) On Ketuvot 71a we have a dispute between Rav and Shmuel regarding the
interpretation of the difference between Rabbi Yehuda and Tana Kama in our
Mishna on Daf 70a.
The Gemara asks: "Didn't we learn this already" - regarding a disagreement
between Rav and Shmuel with respect to another Mishna, in Nedarim.
What does the Gemara mean to imply by this question? After all, Rav tought in Sura and Shmuel in Naharda'a - wouldn't we expect both teachers to teach and interpret all of the Mishnayot, each in his respective Yeshiva when he taught the Mishnayot of that Massechet?
Could it be that the question of the Gemara is: Why, IN OUR TEXTBOOKS - that are taught in the days of the later Amoraim, or in the days of the Sevoraim, is this dispute reported twice? Implying that the students in the latter day Yeshivot could have been expected to extrapolate the dispute learned in one Massechet to the other.
But surely the question cannot be interpreted to ask why did Rav and Shmuel teach the different Mishnayot in different places!
As you point out, the question is why it was taught as a Machlokes between Rav and Shmuel if we already learned that they disagree on this point. Of course, as you said, each one had to teach the Halachos individually.
D. Z.