The issue isn't that the RNG is flawed so much as you ran in 6 times out of the dungeon being ran X amount of times across all the servers in the time you did 6 runs, meaning a large number sets of items dropped during that time. At that point, you getting the same items is more common than you think if they are dropping 12% of the time and the other 88% have already happened between each run. Insert more imaginary numbers to further prove point.