Trying to make a 1.14 mod, got an error: non-unique class target name mappings

[u9g]

gradle.properties:

# Done to increase the memory available to gradle.
org.gradle.jvmargs=-Xmx1G
# Fabric Properties
# check these on https://modmuss50.me/fabric.html
minecraft_version=1.14
yarn_mappings=1.14+build.21
loader_version=0.13.3
# Mod Properties
mod_version=1.0-SNAPSHOT
maven_group=dev.u9g
archives_base_name=minecrft-data-generator-server
# Dependencies
# check this on https://modmuss50.me/fabric.html
fabric_version=0.28.5+1.14

Error:
https://gist.github.com/u9g/e289e2437b6eddf4e2dc9b7adc940390

How can I fix this?

[modmuss50]

In your build.gradle make sure its not using the :v2 mappings. There is a bug with the retro generated v2 mappings for 1.14 causing them to not work as expected.

mappings "net.fabricmc:yarn:${project.yarn_mappings}:v2"

Should become:

mappings "net.fabricmc:yarn:${project.yarn_mappings}"

[u9g]

Works great, thanks!

[u9g]

Okay wait, now I can't use any net.minecraft classes

[u9g]

False alarm, just had to run genSources