Alchemy should only include effects shared by at least 2 ingredients
That is, in vanilla, if an effect appears twice but only in the same ingredient (Ash Salts are the only example without mods) it isn't supposed to appear in the potion. If this was intended, then you should probably allow single-ingredient potions as long as they produce an effect.
Also trying to make a potion with no effects you get a "potion failed" but it doesn't consume any ingredients.
(RM-2722 from redmine: created on 2015-06-23 by Ohgod Noway, , closed on 2015-06-24 by scrawl .)