> Considering in the Google antitrust case it came out that the companies were working hand in glove for years, what were have is a duopoly where the participants collude.

But then, the problem is that you have a cartel, not a duopoly. That’s the thing: you can only punish companies for their actions. A duopoly is a fact, in itself it does not imply any particular behaviour from either company. If there is collusion, then it’s anti-competitive behaviour, abuse of their dominant positions in the market, etc. Things that are already illegal and should be enforced.