Commentary for Bava Metzia 3:10

But doesn’t it teach, "This one," and, "This one": Concerning, "I found it," it taught, "This one said"; and concerning, "It is all mine," it taught, "This one said." And if it was one, this is how it should have taught: This one says, "I found it and it is all mine."

But doesn't [the Mishna] teach: this [one] and this [one]? The Gemara suggested that even though the Mishna uses the phrase אני מצאתיה - I found it, and the phrase כולה שלי - it is all mine, it is not speaking of two different cases. Each of the litigants is simply saying - I found it and it all mine. The Gemara then rejects this suggestion because the Mishna did not need to repeat the words זה אומר - this one says, twice. The Gemara then concludes that the Mishna is in fact speaking of two distinct cases.
Tosfos comments: There are places throughout the Talmud, that [the Gemara] asks this question: Why did the Mishna need to repeat “this one says”? There are places where [the Gemara] does not ask this question. [See Tosfos Bechoros 31b 7-8 (ד'ה אם כן] where Tosfos elaborates on this subject and cites many more instances throughout the Talmud. Tosfos does suggest some reason for this inconsistency.