[Arijharn]

I think it's tied to Bloodcrushers (not Bloodletters) of Khorne, that if I add a Herald to it, add some gifts and delete the unit (while another Bloodcrusher unit with or without Herald exists) I get the error.

I just duplicated it again; try this example:
Army Builder v3.3b #243

Made an army list; Daemons of Chaos - 3000points

Went to Rare choices tab (to order list); chose Bloodcrushers (double-clicked), pressed the plus button so that it reads 7 Bloodcrushers (I believe in the whole Khorne 'magic number thing' don't want to offend the Gods of Chance!)
Went to Heroes tab
Chose Herald of Khorne, clicked and dragged hero into the Bloodcrusher's unit, gave him a Juggernaut mount, gave him Etherblade gift.
--
went back to Rare choices, double-clicked another unit of Bloodcrushers; validation error message of Duplicate Special Characters.

I guess this means it's not limited to deleting the unit while other's present, maybe it occurs if two units of Bloodcrushers are next to each other on the list? I'll try that right now: yep. Two Bloodcrusher units appearing in sequential order gives me this message.