[1.10] Mapping source name conflicts in common project for 1.21.1

[benjaminiserman]

This is a Kotlin Neoforge 21.1.152 + Fabric 0.16.14 project for Minecraft 1.21.1. I'm currently upgrading from a working 1.20.1 version.
The common project also depends on Forge Config API Port 21.1.3 and Architectury API 13.0.8, and has optional dependencies for CC: Tweaked, Create, and Ponder.

I am using only Mojang's loom.officialMojangMappings().

Things I've tried to fix this error:

• ./gradlew --refresh-dependencies
• ./gradlew genSources
• ./gradlew clean
• rm -rf all .gradle and build files in all subdirectories
• downgrade Loom to 1.7-SNAPSHOT (Could not create an instance of type net.fabricmc.loom.extension.LoomProblemReporter.)
• downgrade NeoForge to 21.1.100
• downgrade Forge Config API Port to 21.1.0
• ./gradlew migrateMappings --mappings "1.21.1+build.1" --input common/src/main/kotlin/
• rewriting all of my classes that appear in the error log (successfully removed my classes from the error log, but did not resolve Minecraft's own methods conflicting with themselves)

I wonder if maybe the migrateMappings script doesn't work on Kotlin projects? Not sure how I can resolve that if so, but I'm confused that wiping the loom caches didn't seem to fix anything either.

Relevant files:

• common build.gradle
• overall build.gradle
• gradle.properties

Error logs:

> Task :common:remapJar FAILED
Mapping source name conflicts detected:
  net/minecraft/world/SimpleContainer METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/SimpleContainer/method_59984]
  net/minecraft/world/level/block/entity/CrafterBlockEntity METHOD isEmpty (()Z) -> [net/minecraft/world/level/block/entity/CrafterBlockEntity/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/level/block/entity/CrafterBlockEntity METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/level/block/entity/CrafterBlockEntity/method_59984]
  net/minecraft/world/inventory/MerchantContainer METHOD isEmpty (()Z) -> [net/minecraft/world/inventory/MerchantContainer/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/inventory/ResultContainer METHOD isEmpty (()Z) -> [net/minecraft/world/inventory/ResultContainer/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/level/block/entity/BaseContainerBlockEntity METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/level/block/entity/BaseContainerBlockEntity/method_59984]
  net/minecraft/world/item/crafting/RecipeInput METHOD isEmpty (()Z) -> [net/minecraft/world/item/crafting/RecipeInput/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/level/block/entity/ChiseledBookShelfBlockEntity METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/level/block/entity/ChiseledBookShelfBlockEntity/method_59984]
  net/minecraft/world/level/block/entity/LecternBlockEntity$1 METHOD isEmpty (()Z) -> [net/minecraft/world/level/block/entity/LecternBlockEntity$1/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/entity/vehicle/AbstractMinecartContainer METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/entity/vehicle/AbstractMinecartContainer/method_59984]
  net/minecraft/world/level/block/entity/ChiseledBookShelfBlockEntity METHOD isEmpty (()Z) -> [net/minecraft/world/level/block/entity/ChiseledBookShelfBlockEntity/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/entity/player/Inventory METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/entity/player/Inventory/method_59984]
  net/minecraft/world/level/block/entity/RandomizableContainerBlockEntity METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/level/block/entity/RandomizableContainerBlockEntity/method_59984]
  net/minecraft/world/inventory/TransientCraftingContainer METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/inventory/TransientCraftingContainer/method_59984, net/minecraft/world/Container/method_5438]
  dev/biserman/wingscontracts/block/ContractPortalBlockEntity METHOD isEmpty (()Z) -> [dev/biserman/wingscontracts/block/ContractPortalBlockEntity/method_59987, net/minecraft/world/Container/method_5442]
  dev/biserman/wingscontracts/container/AvailableContractsContainer METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, dev/biserman/wingscontracts/container/AvailableContractsContainer/method_59984]
  net/minecraft/world/level/block/entity/LecternBlockEntity$1 METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/level/block/entity/LecternBlockEntity$1/method_59984]
  net/minecraft/world/inventory/MerchantContainer METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/inventory/MerchantContainer/method_59984]
  net/minecraft/world/SimpleContainer METHOD isEmpty (()Z) -> [net/minecraft/world/SimpleContainer/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/CompoundContainer METHOD isEmpty (()Z) -> [net/minecraft/world/CompoundContainer/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/item/crafting/CraftingInput METHOD isEmpty (()Z) -> [net/minecraft/world/item/crafting/CraftingInput/method_59987, net/minecraft/world/Container/method_5442]
  com/simibubi/create/foundation/blockEntity/LegacyRecipeWrapper METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, com/simibubi/create/foundation/blockEntity/LegacyRecipeWrapper/method_59984]
  com/simibubi/create/foundation/recipe/DummyCraftingContainer METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, com/simibubi/create/foundation/recipe/DummyCraftingContainer/method_59984]
  net/minecraft/world/inventory/ResultContainer METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/inventory/ResultContainer/method_59984, net/minecraft/world/Container/method_5438]
  net/minecraft/world/ticks/ContainerSingleItem METHOD isEmpty (()Z) -> [net/minecraft/world/ticks/ContainerSingleItem/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/entity/player/Inventory METHOD isEmpty (()Z) -> [net/minecraft/world/entity/player/Inventory/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/inventory/TransientCraftingContainer METHOD isEmpty (()Z) -> [net/minecraft/world/inventory/TransientCraftingContainer/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/level/block/entity/RandomizableContainerBlockEntity METHOD isEmpty (()Z) -> [net/minecraft/world/level/block/entity/RandomizableContainerBlockEntity/method_59987, net/minecraft/world/Container/method_5442]
  com/simibubi/create/foundation/recipe/DummyCraftingContainer METHOD isEmpty (()Z) -> [com/simibubi/create/foundation/recipe/DummyCraftingContainer/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/Container METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/Container/method_59984]
  net/minecraft/world/item/crafting/CraftingInput METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/item/crafting/CraftingInput/method_59984]
  net/minecraft/world/CompoundContainer METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/CompoundContainer/method_59984]
  dev/biserman/wingscontracts/block/ContractPortalBlockEntity METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, dev/biserman/wingscontracts/block/ContractPortalBlockEntity/method_59984]
  net/minecraft/world/entity/vehicle/ChestBoat METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/entity/vehicle/ChestBoat/method_59984]
  net/minecraft/world/entity/vehicle/ContainerEntity METHOD isEmpty (()Z) -> [net/minecraft/world/Container/method_5442, net/minecraft/world/entity/vehicle/ContainerEntity/method_59987]
  com/simibubi/create/foundation/blockEntity/LegacyRecipeWrapper METHOD isEmpty (()Z) -> [com/simibubi/create/foundation/blockEntity/LegacyRecipeWrapper/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/item/crafting/SmithingRecipeInput METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/item/crafting/SmithingRecipeInput/method_59984]
  net/minecraft/world/item/crafting/SmithingRecipeInput METHOD isEmpty (()Z) -> [net/minecraft/world/item/crafting/SmithingRecipeInput/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/ticks/ContainerSingleItem METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/ticks/ContainerSingleItem/method_59984]
  net/minecraft/world/level/block/entity/BaseContainerBlockEntity METHOD isEmpty (()Z) -> [net/minecraft/world/Container/method_5442, net/minecraft/world/level/block/entity/BaseContainerBlockEntity/method_59987]
  net/minecraft/world/Container METHOD isEmpty (()Z) -> [net/minecraft/world/Container/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/item/crafting/RecipeInput METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/item/crafting/RecipeInput/method_59984]
  dev/biserman/wingscontracts/container/AvailableContractsContainer METHOD isEmpty (()Z) -> [dev/biserman/wingscontracts/container/AvailableContractsContainer/method_59987, net/minecraft/world/Container/method_5442]
  net/minecraft/world/item/crafting/SingleRecipeInput METHOD getItem ((I)Lnet/minecraft/world/item/ItemStack;) -> [net/minecraft/world/Container/method_5438, net/minecraft/world/item/crafting/SingleRecipeInput/method_59984]

Notably every mapping issue seems to be related to net/minecraft/world/Container, of which I have three overriders AvailableContractsContainer directly, ContractPortalBlockEntity via WorldlyContainer, and CompactingContainer via SimpleContainer. Removing the erroring overrides did not help.