Building this contraption I have shown below with the command blocks reading "/testfor @p[r=4]". It properly dispenses when inside the radius of 4 but when you leave a radius of 4 the dropper dispenses once again. It appears that it is the dropper dispensing when it's loosing power or that the comparator is ignoring that the command block should not be outputting power. My build may be the cause of the problem.