A mod can be loaded only once. When the mods folder holds two files of the same mod, often an old version and a new one, Forge and NeoForge stop at start with Found duplicate mods. Keep one file and remove the other.
What you see
The Why your server stopped box on Overview and above the Console says Two copies of and the mod's id are installed., then Keep one of these and remove the other from the mods folder: and the two file names. The button Open in Files opens the mods folder.
In the Console, the loader prints Found duplicate mods:, then a line starting Mod ID: with the mod's id and from mod files: with both file names.
Fix it
- Click Open in Files in the box, or open Files and the
modsfolder. - Find the two files the message names. They often differ only by their version number.
- Keep the one made for your Minecraft version, usually the newer one. Delete the other, or rename it so it ends in
.disabled. - Start the server.
Why it happens
- A mod was updated by uploading the new file, and the old file stayed.
- The same mod was installed twice, once from Modding and once by hand.
- A modpack already contains the mod, and it was added again.
Updating from Modding replaces the old file once the new one is in place, so this does not happen there: How to install mods on Forge, Fabric and NeoForge.
Good to know
- Plugins have the same problem on Paper, Purpur and Spigot. The console then says
Ambiguous plugin namewith two file names: delete the older file frompluginsin Files, then restart. - Other causes the panel names: How to see why your Minecraft server stopped.
