The mod is using outdated Tags and causing a duplicated entry. The correct tag id is "c:foods/vegetable" (not plural).
https://github.com/Sinytra/ForgifiedFabricAPI/blob/1.21.1/fabric-convention-tags-v2/src/generated/resources/data/c/tags/item/foods/vegetables.json

Carrots, Potatoes, beets, and Golden Carrots all have duplicated tags c:foods/vegetable and c:foods/vegetables

These are not just the only ones. Many other tags have changed and it's causing the duplicated tags.
https://github.com/neoforged/NeoForge/tree/1.21.x/src/generated/resources/data/c/tags/item

This seems to be consistent with upstream FAPI' tags. Are there any in-game side effects caused by this that would require attention?

Yes, adding c:coal to the #minecraft:coals tag is breaking some of my recipes

This is causing the c:coal tag to be added to charcoal and coal coke items from other mods I have. It breaks the recipes that use the c:charcoal and c:coal_coke tags. To fix that, you just have to unappend the c:coal from the #minecraft:coals tag and instead add the minecraft:coal item.