Duplicate mods: two jars, one mod ID, no server
Found a duplicate mod and Duplicate mod ID errors mean two jars claim the same mod. The loader will not pick one for you. Here is how to find and clear them.
The short version
Duplicate mods happen when two jars in the mods folder declare the same mod ID, usually the same mod at two versions after an update where the old jar was never deleted. Forge and Fabric both refuse to start rather than guess. Delete the older jar and restart.
This one is almost always self-inflicted, easy to fix, and easy to avoid entirely once you know the cause.
#What it looks like
Forge and NeoForge:
Found a duplicate mod examplemod at ...examplemod-1.2.0.jar and ...examplemod-1.3.0.jar
Fabric:
Duplicate mod ID: examplemod
Both mean two jars claim the same identity, and the loader will not choose for you. It is right not to: mods reference each other by ID, and two candidates makes every reference ambiguous.
#Where duplicates come from
Updating a mod without deleting the old jar. The overwhelming majority. Upload examplemod-1.3.0.jar, forget examplemod-1.2.0.jar is still there, restart, crash.
Uploading a folder rather than files. Some upload flows preserve a nested directory, so you end up with the mod both loose and inside a subfolder.
A modpack plus manual additions. You add a mod the pack already includes under a different filename. Same mod, two jars, different names, and nothing obvious to the eye.
Restoring a backup over a modified mods folder. The restore adds its jars alongside what was already there.
#Clearing it
- Open Files and go to
/data/mods. - Sort by name. Duplicates land next to each other because they share a prefix.
- The error names both paths. Delete the older version.
- Restart.
If the error names two jars with entirely different filenames, open both and compare their mods.toml (Forge) or fabric.mod.json (Fabric) for the declared mod ID. Repackaged mods and mods bundled inside other mods both cause this, and the filename tells you nothing useful in those cases.
#Avoiding it
Delete before you upload. Not after, not at the same time: the old jar goes first, then the new one arrives, and there is never a moment where both are present.
If you update several mods at once, take a backup first, then clear all the old jars and upload the new set in one go. Trying to update in place, one file at a time, in a folder with two hundred jars, is how mods folders end up in the state that produces this error.
#While you are in there
A mods folder that has accumulated duplicates has usually accumulated other things: mods nobody remembers adding, mods disabled by renaming to .jar.disabled two months ago, leftovers from a pack you moved away from.
Each of those costs memory and startup time whether or not anyone uses them. Clearing them out is not urgent, but a duplicate-mod error is a reasonable moment to spend ten minutes on it.
#Frequently asked questions
What does Found a duplicate mod mean?
Two jar files in your mods folder declare the same mod ID. Almost always this is one mod at two versions, left behind because an update was uploaded without deleting the previous jar. The loader stops rather than guess which you meant.
Why does updating a mod cause duplicates?
Because uploading a new jar does not remove the old one. The mods folder ends up with both, they declare the same mod ID, and the loader refuses to start. Deleting the old jar before uploading the new one avoids it entirely.
Can two versions of the same mod ever coexist?
No. Mod IDs are unique by design, because mods reference each other by ID and two candidates makes every reference ambiguous. One of the jars has to go.
Written and maintained by the Hostd engineering team. Last updated 2026-08-05. Notice a mistake? Tell us.