I have identical twin boys. They shared a placenta so defintely had the same pre bith experience!
They were 8 weeks prem, so 40 years ago may not have survived.
They have severe asthma and allergies BUT their allergies are not all the same.
DT 1 has peanut allergy, DT 2 does not.
DT 2 has dust mite allergy, DT 1 does not.
Both have milk, wheat, soya and egg allergies, all proven by blood test, so not my over diagnosing or intollerances. They are not anaphalactic and as they have grown have been able to have soya, eggs and wheat and some dairy. But we have found dairy causes problems with their asthma.
At a few hours old DTs were both given formula through NG tube as I had not yet expressed enough milk. DT 2 coped fine, DT 1 did not and he could not digest the formula.
So, to recap - ID twins. Same upbriging. Same pregnancy. Different allergies. One showing signs of problems with dairy at a few hours old.
IMO I cannot see what I could have done to change this. More to the point, I think my children's bodies were too young to cope with being born, and therefore were left with asthma and allergies. Not because I cleaned my house too much/ too little, because I ate the wrong foods or because I weaned too early or too late.
It may also be genetic. I have hayfever, DH has eczema. But my dad had problems with dairy as a child, but just had to get on with it. Diagnosis wasn't as common then.
BTW DS1 has very very mild asthma and no allergies.