Armors don't actually get doubled that often by fodder mages. We've had this discussion before. The issue is that Fire Emblem incentivizes certain unit characteristics and armors by design lack almost all of these incentivized characteristic. Really their only meta trait is defense, but it's not enough to salvage them. FE gameplay is genuinely not complex enough for the role diversity that armor knights insinuate. It's a fast paced tactics game about covering ground and killing lots of enemies on their own turn. Armors don't fit in well with this schema.