3* : I plan on having 2x Balthazar, 3x Bane, and 2x Squire Wabbit. I might even use 3x Wabbit if I had a third.
4*: 2x Jackal and Falcon work great as mentioned above. I’d also consider 2x Tiburtus and Grimm since nearby hitters are great in events. I omit Gormek since his offense is weak and there are many grea 4* reds.
5*: These are less applicable since duplicates and mats are harder to come by but: 2x any of the single target heavy hitters would work well - Magni, Lianna, Sartana, Marjana, Joon. Panther, Arthur, and even Kestral are also good enough to consider doubling. Since I know people will call me out on Kestral, his increased damage vs high mana targets can be devastating on level 10 vs 3 bosses, and even on 6-9 vs 2. He’s not great in other contexts so I don’t actually expect anyone to level him 2x though.
Edit: I realize I was answering the question of what heroes you would run doubled in an event. All the 4* heroes except Boomer are good enough to want doubled. I don’t think I’d run more than 1 copy of any 5* except Panther or Arthur.