That's terrible to hear. I've never heard of somebody working a "double shift", unless it was another, unrelated job, before doing the carrier job. 12 hours were the max, even in the old days, and now they have almost done away with overtime, which some workers used to depend on. Which is what made me think of a second job.
My bosses were always good at times like this - they'd get a case of cold water from Wawa, or somewhere like that, and bunch of ice, and it would be in a huge ice chest, for us to grab one, when leaving. And they'd go around checking up on us, in the afternoon. And the ones they knew to check up on the most were the new people - they always had a lot in the summer, to fill in for vacationers, and even though most were young, it's easy to overdo it in the heat, if not used to it. Most of us would bring a small ice chest with a half dozen bottles of frozen water, and drink them as they thawed. Plus, I had a lot of nice people on my route, that were always checking on me! They got treats from my garden, if I found out I grew something they liked. All those years in the summer heat, and I'd still come home and do what I had to in the garden! I got used to the heat, even though I hated it.