Here's the list of what each person requires for each custom item. Asterisks (*) mean the item seems to be unique in the game:
- Antone
- Steelhide Armor - Steelhide Croc Hide*, Poison Seeker Sac, Iron Weed Thorn
- Beastslayer - Piercer Modai Claw, Wide Cleaver Blade, Giant Picus Egg
- Zynaryx Plate (Faeries Only) - Venom Crab Shell, Tomb Skull,
Giant Spider Silk
- Featherweight Armor - Tanto Wasp Wings, Plumed Serpent Feather, Djinn Eye*
- Ebon Staff - Wood Spirit Staff, Ebon Gem*, Mummy Dust
- Ferro
- Mirror Armor - Giant Silver Nugget*, Obsidian Piece*, Locket of Reflextion
- Vampire Chain - Vampire Bat's Wing, Prisoner's Chain, Heart of a Sorceress*
- Ivory Blade - Hogar Tusk, Piece of Coral*, Ring of Protection
- Fuzzfas -
- Earthquake Potion - Blue Feather*
Also - to the best of my knowledge each armorer will only create one of a custom item even if you bring him all the ingredients to make a second one. At least, when I tried to give Antone a 2nd venom crab shell and tomb skull, he wouldn't take them. That might only happen if he hasn't completed the first item yet - I never did manage to get the spider silk to get my first set of Zynaryx Plate made. Perhaps if I had he would have accepted a commission for a 2nd set. In any case, the Zynaryx Plate and Beastslayed appear to be the only ones you have a possibility of getting a second copy of and the Beastslayer isn't worth it.
Ah -- well that would explain things then. I didn't know about the Djinn eye. The only place I encountered Djinn was the one in Mt. Gigas and then I ran into them on Ascension Peak but only the one in Gigas dropped an eye so I just figured it was a singleton item too. Good to know about the Featherweight. I'll keep that in mind for this time through. 
BTW - just like the Beastslayer isn't much good, neither is the Steelhide armor, IMO. Well, it IS considered Medium grade so you can have rogues/gadgeteers/bards/rangers wear it and get an AC as good as a F/V/L wearing plate mail but it's pretty darn heavy - 26 lbs - so I don't consider the trade-off to be worth it considering that normally my Medium-armor characters have lots of items/ammo they need to carry around and I'd rather put them in Featherweight or Chamail+2 (an EXCELLENT non-magical armor, BTW, and both importable from 7 as well as available pretty early in the game. I think it is way undervalued as a decent armor).
For my money, I prefer the Flak Jacket/Vests and Chamail+2 for any medium non-class-specific armor. Now that I know about the Featherweight, it might replace these in my preferences, however.