Enrichment does not stack
The exact text of the 8-hour version of Plant Growth is:
If you cast this spell over 8 hours, you enrich the land. All plants in a half-mile radius centered on a point within range become enriched for 1 year. The plants yield twice the normal amount of food when harvested.
Importantly, the direct effect of the spell is to enrich the land and the plants growing on it, not to double the harvest. The first time you cast the spell, the plants within the radius become enriched. If you cast it again in the same place, there is no effect, because the plants are already enriched. No matter how many times you enrich the plants, they are still just enriched. The enriched plants, when harvested, yield double the normal amount.
(Note: You can arrive at this same conclusion using the DMG's rule for combining game effects, but my answer here doesn't rely on this rule or any particular interpretation thereof. Even if effects of the same name could stack, enrichment as described in the spell's text is a binary state that cannot stack and which can only cause plants to produce "twice the normal amount", which is independent of how many times the spell is cast.)