I have many duplicates to be honest and that was mostly for gauntlet but also for LOL and missions where you need ot hit targets and where the dupes can help (if that makes sense). However, now that I have played gauntlet I am not sure how useful they are to be honest - I rarley want or need to go beyond SSS/SS so dupes are not a big help really - so I may be getting rif of them over time at sac events but keeping them for now. I also have dupes of characters that can do the six sets or have useful skills like immunity/block like others have said. I also tend to keep dupes of the characters I like to play or who are good in LOL/BNW - so corrupt rescuers, hellraisers, demon spawn and characters like Prisoner, G COTD, Witch Doctor, Alliance General......
Not surte if they wil stick but for now I am keeping them and now that the roster size has expanded that is fine though I now need to add in lower level characters to complete my full roster as I got rid of a fair few when I ran out of room.
I am slowly upgrading the dupes for either use or sac.