Personally I figure if I already have two copies of a rare (or epic I guess, but that hasn't happened yet :D ), the third or any additional ones can serve as skill books. In some cases already the second copy. And with the rares that drop from farming the campaign, after I fully upgraded all skills on my Diabolist, I just use them as leveling food - I'm bound to get plenty more of them in future farming. 

But I wouldn't do that with heroes that I have just a single copy of, no. They might get revamped later on, or they might become necessary for a fusion. Nothing more frustrating than having to grind for a hero which you once had but used as food. 

Regarding the three heroes you mention: Torturehelm is needed for the Rhazin Scarhide fusion, so definitely keep him unless you already have Rhazin. I wish I had him so I could fuse Rhazin... And Aina and Ursine Ironhide don't seem that useless, based on what I'm reading - Aina has both weaken and decrease def, which you'd think makes her viable for Clan Boss with the right team, while Ironhide has decrease attack and a nice turn meter manipulation passive. 

Id say Aina migth be useful early on. If I started a new account I  would probaly be very hesistant with leveling her to level 60 and definiitely save my books. She is not reliable for decrease defense because of long cooldown. In generel I dont like attack heroes unless they bring something awesome to the team.