The enrichment does not pile up
The exact text of the 8-hour version of Plant growth is:
If you cast this spell for more than 8 hours, you enrich the country. All plants located within a half-kilometer radius centered on a point located in the beach enrich for a year. The plants give twice the normal amount of food during the harvest.
Especially the direct The effect of the spell is to enrich the earth and the plants that grow there, and not to double the harvest. The first time you cast the spell, the plants in the combo enrich. If you put it back in the same place, there will be no effect because the plants are already fortified. No matter how many times you enrich the plants, they only get rich. The enriched plants, once harvested, give twice the normal amount.