Yes, when crimes are committed it is often hard to prove and you won't catch them all. That's by design and a fundamental part of how our legal system was designed in the first place.

Having a duopoly as you described isn't in itself a crime, nor should it be. If they are skirting the rules such that they are breaking the rules, enforcement should step in as there is actually something to enforce. If the only "crime" is winning market share, what's the problem?