I'm currently using this cmd: "/spreadplayers 6 -1878 1 17 false @e[type=Squid,r=8]" and it spreads squids on to 1x1 platforms that are in a grid above lava and sometimes it will spread 2 squids on the same platform.
EDIT: Also happens when not above lava, when just having the command blocks.
(Still in 1.8.2 pre-6)
I just found out why you can have 2 entity on the same block even with a spreadDistance of 1 : The game pick a random FLOAT coordinate, and do many iterations in order to find a solution. Then he center every entities on the block where they stand. The problem is that 2 entities can be away from more than 1 block and be on the same block (2 opposite corner of a square are far from sqrt(2) > 1)
If you don't want any entity on the same block, put a spreadDistance of 1.42