> mentioning both only makes sense if they are different.

I disagree. Stating the `contact` address explicitly is to say with certainty that it is known that it is the correct contact address. If one only has the building address, a user consuming the data won't know for sure if the contact address is the same as the building address — they can only guess that it is the case; however, it's also possible that it was simply unknown to the mapper at the time what the correct contact address is, and it could therefore very well be different than the building address if it wasn't mapped.