For F2P players who only have slim chances to get great event or Atlantis heroes twice, the best choices for dupes from TC13/20 heroes in my eyes are:
Red: Definitely Boldtusk (awesome for wars, I almost always use BT if there are red heroes in my team, so a second or even third always comes handy). Maybe Scarlet and Gormek. Rather not Colen or Kelile.
Blue: Kiril (see BT), Grimm (great blue hero in all aspects of the game; even pretty nice as a dupe in one attack team - for epic difficulty in monthy quests I often use both my Grimms, when I go with blue mono-teams 2 Grimms also pay off if no or few fully ascended 5* heroes are available) Maybe Sonya, definitely not Boril.
Green: maybe Caedmon or Gandalf; Skittleskull if no other option is available; definitely not LJ or Kasshrek
Yellow: Li Xiu or Chao as additional mana controlers; as for Wu Kong, I’m not fond of him apart from Titan battles; as for the panda, don’t even bring him to fourth ascension level…
Purple: definitely Rigard (as F2P player you usually lack cleansers); maybe Tiburtus or Sabina and definitely not Cyprian.