Yes, the dispensers seemed to go off twice for each bucket I poured into the cauldron (and the cauldron looked like it went down by halves instead of thirds). In case it makes a difference, I'm doing this in single-player survival with GT5u, and I built the underlying structure with cobblestone instead of iron blocks or that one piece of red wool in your images (which I don't understand).
The dispensers would go off a 2nd time because they ticked again before it caught that the cauldron was empty. Try replacing the Redstone in front of the comparator with a solid block and see if that adjusts it enough.
It shouldn't make any difference if you're utilizing cobblestone; and as for the Red Wool, it doesn't matter what block you place there. It's intended to make sure the redstone doesn't loop with itself, and I made it red so it draws attention.
Good Luck,