Having two of the same feature in the same step causes a world crash instead of a failed datapack validation. ex (note the double use of the glow lichen feature):

 

 "features": [
    [],
    [
      "minecraft:lake_lava_underground",
      "minecraft:lake_lava_surface"
    ],
    [],
    [],
    [],
    [
      "minecraft:monster_room",
      "minecraft:monster_room_deep"
    ],
    [
      "minecraft:ore_dirt",
      "minecraft:ore_gravel",
      "minecraft:ore_granite_upper",
      "minecraft:ore_granite_lower",
      "minecraft:ore_diorite_upper",
      "minecraft:ore_diorite_lower",
      "minecraft:ore_andesite_upper",
      "minecraft:ore_andesite_lower",
      "minecraft:ore_tuff",
      "minecraft:ore_coal_upper",
      "minecraft:ore_coal_lower",
      "minecraft:ore_iron_upper",
      "minecraft:ore_iron_middle",
      "minecraft:ore_iron_small",
      "minecraft:ore_gold",
      "minecraft:ore_gold_lower",
      "minecraft:ore_redstone",
      "minecraft:ore_redstone_lower",
      "minecraft:ore_diamond",
      "minecraft:ore_diamond_large",
      "minecraft:ore_diamond_buried",
      "minecraft:ore_lapis",
      "minecraft:ore_lapis_buried",
      "minecraft:ore_copper",
      "minecraft:underwater_magma",
      "minecraft:disk_sand",
      "minecraft:disk_clay",
      "minecraft:disk_gravel",
      "minecraft:lush_caves_clay",
      "minecraft:ore_debris_small"
    ],
    [],
    [
      "minecraft:spring_water"
    ],
    [
      "minecraft:glow_lichen",
      "minecraft:glow_lichen",
      "minecraft:bamboo_light",
      "minecraft:trees_jungle",
      "minecraft:flower_warm",
      "minecraft:patch_grass_jungle",
      "minecraft:brown_mushroom_normal",
      "minecraft:red_mushroom_normal",
      "minecraft:patch_sugar_cane",
      "minecraft:patch_pumpkin",
      "minecraft:vines",
      "minecraft:patch_melon"
    ],
    [
      "minecraft:freeze_top_layer"
    ]
  ]

 

In 1.17 this was not a problem but apparently this is why it keeps failing in this new version. The error is as follows:

Feature order cycle found, involved sources: [Reference

{ResourceKey[minecraft:worldgen/biome / minecraft:windswept_savanna]=cgw@485aaa94}

, Reference

{ResourceKey[minecraft:worldgen/biome / minecraft:lush_caves]=cgw@32e72c5d}

]