It seems Environmental is redundantly adding Deer to Twilight Forest that already have Deer mobs. This issue seems to stem from Environmental searching all biomes that are a forest in the Biome dictionary, and adding them to the spawn tables.

Since it's not exactly possible to detect what biome a feature is generating within, but it's possible to filter what gets modified into a biome during the biome registration event (or something like that, I'm only familiar with 1.16.2+ Biome creation, not modification). A configuration blacklist would be beneficial to prevent future redundancies.

If you wish to even add Twilight Forest biomes into your default blacklist array, our biomes are listed here complete with biome IDs

Re-directing to #240