Do Salt resources tend to clump like that? I think my current game is the only one where I've seen a civ have 13 copies of a resource.
I think the only luxury that never comes in cluster is marble (I never saw more than one in the same area in my games)
Also, I can't recall whether I've seen a fur cluster either.
Whales don't cluster either, at least I haven't seen whale clusters. Also, I can't recall whether I've seen a fur cluster either. But excluding jewelry, porcelain, nutmeg, cloves and pepper for obvious reasons, those are the only non-clustering luxuries I can think of.
I love salt, it's my favorite luxury resource. But I would say I start close to it 5-10 % of the time. I think the only luxury that never comes in cluster is marble (I never saw more than one in the same area in my games)