I was thinking of arrays of objects, where the keys are repeated for each object in the array.

I'm still a bit confused how this is a problem. Can you elaborate?

Is there any serialisation standard that specifically addresses this? I know YAML and TOML doesn't at least (correct me if I"m wrong)

Something like Protobuf where there is a schema is defined means metadata like key name does not get repeated for every object, its only the values.

No more trying to have short names to keep file size down.

IMHO, it sounds like the job of the compression algorithm, not the format.

That's what I said in my first comment, no?

s/arrays/"collections of homogenous objects" and it's a fair point.