Duplicate mod found / mod id already present
A mod id is a unique identifier inside each jar. When two files declare the same id — typically alpha and updated versions of the same mod, or a mod bundled inside both a pack and a library — the loader refuses to pick a winner and lists the duplicates at boot.
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How to fix it
- 1
Read the duplicate list
The error names the mod id and usually both file paths. If it names only the id, search the mods folder for the id string.
- 2
Keep the newer copy
Compare file names for version numbers and keep the one matching your pack's manifest. When unsure, reinstall the pack — it restores the matched set.
- 3
Check for bundled duplicates
Some packs ship a library mod both standalone and inside another jar. The standalone copy is the duplicate to remove — the manifest decides which belongs.
- 4
Restart and confirm
One jar per mod id loads cleanly. If the same id reappears after updates, an updater is re-adding the old file — clean the folder before updating.
Alternative causes
These can produce the same error message — worth ruling out if the steps above don't resolve it.
Same mod for two loaders
A mod's Forge and Fabric builds often share one id. Having both jars present triggers the duplicate error even though they are different files — keep only the loader-matching one.
Case-sensitivity near-misses
Two files differing only in capitalization are different files on Linux but look identical in some panels. List the directory to spot them.
Frequently asked
Why won't the loader just use the newest version?
Because guessing violates reproducibility: two files with one id is a broken install state, and silently picking would hide real conflicts. The loader stops and tells you instead.
Can two different mods share an id legitimately?
No. The id is the namespace for configs, recipes and network channels. Two mods with one id would corrupt each other's data even if loading were allowed.
I deleted one and now a dependency is missing — why?
You kept the standalone copy of a bundled library. Reinstall the pack from the manifest; it restores exactly the files the author tested.
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Last reviewed 2026-09-06. If a step is wrong or out of date, tell us — we'll fix the article and the auto-pattern at the same time.