The Issue

Basically it's all the in the title, Quilt Loom adds Fabric Loader or Quilt Loader to the modRuntimeClasspathMainMapped configuration twice, unsure as to the root cause however if you use Quilt Loader this is kind of a non-issue due to it just ignoring one of the entries.

[14:37:57] [main/WARN] (Quilt Loader/Solving) Ignoring duplicate mod quilt_loader of the same version 0.26.3 loaded from C:\Users\Quinn Semele\Projects\Modding\mod issues.gradle\quilt-loom-cache\remapped_mods\org_quiltmc_quilt_mappings_1_21_1_21_build_18_v2\org\quiltmc\quilt-loader\0.26.3\quilt-loader-0.26.3.jar

However if your using Fabric Loader in a project then trying to launch the game will fail.

Caused by: java.lang.IllegalStateException: duplicate fabric loader classes found on classpath: jar:file:/C:/Users/Quinn Semele/.gradle/caches/modules-2/files-2.1/net.fabricmc/fabric-loader/0.15.11/9c2be53d84ec5c4fb58bb5b8d9c8a629047c9ee9/fabric-loader-0.15.11.jar!/net/fabricmc/loader/api/FabricLoader.class, jar:file:/C:/Users/Quinn Semele/Projects/Modding/mod issues/.gradle/quilt-loom-cache/remapped_mods/org_quiltmc_quilt_mappings_1_21_1_21_build_18_v2/net/fabricmc/fabric-loader/0.15.11/fabric-loader-0.15.11.jar!/net/fabricmc/loader/api/FabricLoader.class at net.fabricmc.loader.impl.util.LoaderUtil.verifyClasspath(LoaderUtil.java:65)

The Impact

Basically I'm a multi-loader dev and I've been trying to switch away from Architectury to native build tools (NeoForge's ModDevGradle and Quilt's Loom) and this issue is blocking me from testing the Fabric release of my mod, and if I switch to Fabric's Loom well then the Quilt release has a few issues too and so basically this being fixed would be great ^_^

Oh I've also made a repo showing off this issue if you think it might aid with debugging efforts: https://github.com/quinn-semele/mod-issues

QuiltMC / quilt-loom

Duplicate loaders on the modded runtime classpath #52

The Issue

The Impact