'Alone Again Or' by JY Saville
Franklin lounged across the padded chairs in the mess room, sharing a joke with Halford and Green. They were winding down after a hair-raising moment with an air circulator pump and had left the ship to Harris. It didn't take much running, it had been built for efficiency – a small craft for quick runs between research stations and mining orbitals, though quick could still mean weeks. Long enough for the effects of being cooped up to kick in.
Haverah Harris slowed down as he neared the mess room. He could hear voices, though all of them must be Dean Franklin because there was no-one else on board. Telling jokes this time, which was better than arguing with himself; the previous week it had looked like it might come to blows. Try explaining that to head office. Still, probably best to give Dean a wide berth for a while longer; Haverah would prefer it if one of them stayed sane.
The room fell silent as they watched Harris slink past the open doorway.
"What's his problem?"
"He's decided we're all figments of his imagination."
"Except me," said Franklin. And laughed.