Duopoly scenarios are why the trifecta of historical anti-trust criteria were one of more of:

   - Monopoly market share
   - Negative impact to customers
     (e.g. higher prices)
   - Collusion to thwart competition

Prosecuted under a variety of laws, but all illegal at scale.

The latter two are more relevant to modern global enterprise competition. A duopoly isn't really optimally lowering prices for anything, as there's limited risk of defection and price signaling is pretty easy.