Singer's generalization principle tells us that "If it is wrong for person A to do X it is wrong for similar people in similar circumstances to do X." I agree that this assumes that we know which features of people and circumstances count when deciding whether they are similar.

But this principle may still be a necessary truth. It seems inconsistent to say that A and B are similar people in similar circumstances but that it is good for A to do X and wrong for B to do X. 

Suppose someone said that two paintings, A and B, are alike except that one is beautiful and one is ugly. Wouldn't it be a contradiction to say that? It seems conceptually impossible.

As far as the principle of consequences goes, I agree with you. As a utilitarian I believe that it is wrong to do things that have undesirable consequences for human welfare. But I don't think that the utilitarian principle or Singer's principle is a necessary truth. I wish I could turn the utilitarian principle into a necessary truth, but I don't think it can be done. 

Couldn't someone deny the principle of consequences and still be consistent? Suppose someone said "It is not true that we ought not to do what has undesirable consequences. We ought to follow God's law. The consequences are of secondary importance."