Will increasing the concentration matter? Obviously I know nothing on the subject, but drinking 10ml of poison in 100ml of water must be as harmful, and thus toxic, to me as 10ml of poison in 50ml water, right?
Or we're assuming they don't drink all the water, in which case 20ml of the first solution will be less toxic than 20ml of the second.
Higher concentration may mean higher rate of absorbtion. If detoxification happens at a constant rate, then a quicker accumulation of toxins may bring you over a threshold you would not’ve gone over had you consumed a poison of lower concentration.
Or we're assuming they don't drink all the water, in which case 20ml of the first solution will be less toxic than 20ml of the second.