Duplicate mods in the pack
Applies to Forge, NeoForge, Fabric, Quilt.
What it looks like in your log
net.minecraftforge.fml.loading.moddiscovery.DuplicateModsFoundException: Found duplicate mods:
What causes it
Two copies of the same mod are installed, so the loader refuses to start.
How to fix it
Reinstall the pack to clear the stray copy, or delete the older duplicate jar from the mods folder.
Why it happens
Two jars in mods/ declare the same mod id. Almost always it is an update where the new jar was added and the old one never deleted, or a pack update that shipped a renamed jar. The loader cannot pick one, so it refuses to start and lists the pair.
Fix it step by step
- 1Read the pair from the errorThe message lists the mod id and both file names.
- 2Delete the older oneVersion numbers are in the file names. Delete the older jar from mods/; renaming it is not enough for every loader.
- 3On a pack, reinstallIf the panel installed the pack, reinstalling it clears stray jars that a manual update left behind.
- 4RestartThe loader should pass mod discovery.
If that didn't work
If the two jars are different mods with the same id, one of them is a fork or a repackaging of the other. Keep the one the pack expects.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log