The npc tag has 55 questions right now, which although could maybe be retagged over some days without too much front-page churn, the system now prevents creating new tags that just have a trailing -s different.
I don't think this is necessarily a good idea.
There are dozens of singular tags just like there are dozens of plural tags. Just going to the front page I see gm-pc, wizard, fighter, npc, and new-gm vs monsters, new-players, skills, and races. We could do a statistical analysis on how many are singular vs plural and then synonymize all of one or the other...
Or we could just get along with our day. The singular or plural made sense to someone and apparently continues to. It does not interfere with finding/applying the tag, searching using the tag, or SEO one bit and I find it hard to justify spending effort to mess with, except for a kind of OCD "wanting things to all look the same." In an emergent folksonomy like site tagging that kind of parallelism will never, and I would argue should never, happen anyway.