Actualy I mean in a straight up duel. Even a spitfire mkVIII will often not be able to pull an instant merge turn much quicker than the 110. So you most like are going to get a second merge coming head to head, possibly a third or fourth before the 110 loses possition. Any of those turns after merge could end in a 30mm for the spit.
If the 110 is attacking then the spitfire is either going to die straight away, or more likely reverse the attack and win comfortably. If the 110 is in defence the spit will win comfortably also in most cases.
But with a co-alt, head on merge, it's up to the 110 to the 110 if its going to be a ho fight. The 110 has the power to say 'hey, lets clean merge this time again' or 'hey, im going to blast you and i dont care if you fire your pea shooters back at me'.