Crocodile
Kuzan
Magellan
Kaidou
Imu
Luffy didn't get a Second Duel against Kuzan or Magellan, only met them again as Obstacles,
His 2nd Duel against Crocodile was after all Baroque Works Agents were already Defeated,
His 2nd Duel against Kaidou was in a completely different Island (Kaidou's HQ) & almost 100 Chapters later,
It should also be noted that this Chapter is 9th of this Current Volume, so this Imu Sub-Plot is wrapping up,
Next Volume will focus on New Sub-Plot (Probably dealing with Gunko, Sommers & Killingham once & for all),
In other words, I don't expect any Power-Up or 2nd Round for Luffy, this was the extent of his Current Power.
All that matters now, is what will Imu decide to do Next?