NPCs don't equip a second ring in some cases
See the discussion in #3754.
Tested in version 0.42. NPCs don't equip a second ring if the first equipped ring has same price, picture, texture and/or model. Andrei Kortunov wrote:
And yes, autoequipping does not take in account that NPC have two ring slots.
This issue can be reproduced with companion, but it affects all NPCs.