It seems to load what would be page 2 now, rather than what was page 2 when you got page 1. So the page 1 stuff has been shuffled downwards, and shows as repeats.
Initial page 1:
Thread a
Thread b
Thread c
Thread d
Thread e
Thread f
Thread g
Thread h
Press more and you get a followup added, of
Thread d
Thread e
Thread f
Thread g
Thread h
Thread i
Thread j
Thread k
But if you'd pressed refresh instead, you'd have got a refreshed Page 1,
Thread 1
Thread 2
Thread 3
Thread 4
Thread 5
Thread a
Thread b
Thread c
and if you immediately pressed "more" then you'd get
Thread d
Thread e
Thread f
Thread g
Thread h
Thread i
Thread j
Thread k
but it would make sense then!