Here it says that Avishai killed 18,000 of Edom in Gei ha'Melach. In Shmuel II, 8:13 it says that David killed 18,000 from Aram there, and Tehilim (60:2) says that Yo'av killed 12,000 of Edom there!
Rashi (Shmuel II, 8:13, from Bereishis Rabah 74:15): We must say that here and in Tehilim discuss different wars.
Radak: The war with Edom was at the time of the war with Aram, like it says (Tehilim 60:2) "b'Hatzoso Es Aram Naharayim v'Es Aram Tzovah... [va'Yach Es Edom]." When it says Aram, this includes those with them (Edom). When it says Edom, this includes those with them (Aram). Initially Avishai killed 6,000, and afterwards Yo'av killed 12,000. It says in Tehilim "va'Yashav Yo'av", for he came after Avishai. Here it attributes the entire war to Avishai, for he began it; all 18,000 are attributed to him. Sefer Shmuel attributes the war to David, for he was primary.
Tanchuma Devarim 3: When Yo'av went to fight Aram, [on the way he encountered] Bnei Edom. They protested - it says "Al Tisgaru Bam" (Devarim 2:5)! Yo'av replied, it says "Atem Ovrim bi'Gevul Acheichem" (Devarim 2:4) - let us pass! They refused. Yo'av reasoned, if we destroy them now, we will not have what to eat and drink on our return [from Aram]. We will leave them until after striking Aram, and then return to fight Edom. Hashem said, what benefit is there to strike Edom bit by bit? Avishai killed 18,000, you killed 12,000 - when the time comes, I will eradicate them! Radak (Shmuel II, 8:13) - according to this, there were two wars at two times. 1 Malbim (ibid.) - it says "va'Yasem Netzivim be'Edom b'Chol Edom Sam Netzivim" (verse 14). I.e. David put commanders, and Edom rebelled, and he conquered all of Edom and put Netzivim in all of Edom.
Malbim (Shmuel II, 8:13): It seems that the war in our verse was at the end, for verse 12 mentioned fighting Amon, which was after [fighting Aram]. Then Aram Naharayim and Aram Tzovah were aroused [to fight] a second time. Edom is near Amon, and came to help Amon. After striking Aram twice, like it says in Perek 10, they struck Aram and Edom together in Gei Melach - 12,000 of Edom and 6,000 of Aram. In Shmuel it says Aram, for the war was primarily against them. Here mentions Edom, for most killed were from Edom. In Tehilim it mentions only the 12,000 from Edom, for it says "b'Hatzoso Es Aram", for this was called on David's name. His two head generals were Yo'av and Avishai. Once [striking Edom] it is attributed to this one, and once to this one. (I.e. Yisrael struck Edom once, and killed 12,000 of them and 6,000 of Aram. - PF) Or, Yisrael fought Edom twice; refer to 18:12:1:2.
