I would say you have found something that is both a bug and working as designed, it looks like each ship has tried to execute the orders given to the task force and in doing so created 8 colonies, instead of 1 colony with 8 times the population. The way around this is probably easiest to establish a colony from the system generation screen F9, just select a body and then add colony, then just use the colony ships for unloading settlers with.

I'm not even sure this could be argued as a bug since I can't think how it could be done so that a TG would do checks, since all ships in the group are making a move at the same time there isn't even an order of movement which could be applied to check for colony existence. The solution to your current issue is either use the ships there in orbit to do load/unload orders of colonists between the new colonies to condense the population to one. Or much quicker just use SM to mod the population of the one you want to keep and then abandon the rest.

I settled on planet with a task force of 8 colony ships and later when viewing the system in the P & P window, the planet was listed 8 times.