The way I understand it, if you cast a low strength, (let's say Enhance INT), then a higher strength one (same spell), the higher intensity takes precedence, but the spell don't add. When the high strength ends, if the lower strength is not finished (because of longer duration), the lower strength is again the one in effect.
If you cast a high strength spell, then a lower strength of the same spell, when the high spell finishes, the lower strength will be the one in effect (with Enhance INT, this could be used to have a longer duration effect, but at a high MP cost).