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.