I have personally done double flushes, two in a evening separated by 4 hours and also one in the evening and one the next morning. I have also done Liver Flushes using large doses of the flush potion up to 14 fluid ounces.
At the time I was impatient to revive my gallbladder and believed I could achieve this by doing more in a quicker time. I reflection with the benefit of hindsight I should have continued at a steady rate because the gallbladder and liver has its own timetable.
Did I get more nausea? No However it is difficult to drink the second dose and it gave me a shudder. It did appear to be more productive but did not necessarily shorten the whole cleansing experience which took months.