Experiment with identical potions, they typically produce essences with their effects disordered, which causes them not to stack, even though they're incongruously the same.

One way to do this would be to sort a collection by their name prior to generating the potion, this will probably cause potion coloration to be somewhat arbitrary. Better would be to sort the collection by potency, so that the most potent effect is always listed first, and then sort by name in the event of a tie. As long as sorts are identical, this should be resolvable.