That they exist as concepts is something they share, I think, with everything else, therefore it isn’t a useful fact.
What is the similarity between an elephant and a plum?
They are both purple, except for the elephant
What did Tarzan say when he saw elephants charging?
“Look out! Here come the plums!” (he was colour blind)
The fact that it is so hard to describe the difference between a lego brick and a sense of duty is exactly that they have so little in common. This is why you say there is no meaningful way in which they differ; this is why the comparison is fairly meaningless to begin with - exactly because comparing requires similarity.
This is in fact the basis of the meaning of ‘comparing apples and oranges’. Of course it is possible to compare apples and oranges - both are fruits that grow on trees; one has a thick orange skin and juicy parts, the other has a thin skin that can have different colors, and is made up of one part. We are now comparing them as fruits. People saying you cannot or should not compare apples and oranges really mean that you should not treat an apple as an orange and describe it as such, saying ‘this orange is odd because it has no thick orange skin, is not made up different parts and is not very juicy’.
In sum, it is a logical precept that in order to accurately describe the difference between a and b, you need to do so in terms of what a and b have in common.
I didn’t think we were finding it particularly hard to describe the difference…