Direct answer

If a mod loader reports duplicate mods, find the repeated mod ID rather than judging only by similar JAR names. Stop the server before moving files.

What it means

Two JARs can embed the same mod ID even when filenames differ; a library or bundled mod may also register content unexpectedly. Duplicate IDs can prevent a loader from constructing a valid mod list.

If you are a player

  • Remove duplicate client mod copies from the correct profile; keep one version that matches the server pack.
  • If a launcher manages the pack, repair/reinstall its official profile instead of mixing downloads.

If you run the server

  1. Read the duplicate mod ID and the two file paths listed by the loader.
  2. Move one conflicting JAR out of the active mods/ directory into a backup folder outside it.
  3. Confirm the remaining file matches the Minecraft version, mod loader and dependency requirements.
  4. Restart in staging and check whether another duplicate ID appears after resolving the first.

Confirm the fix: Do not delete both files or remove a dependency blindly. Save the working mod manifest so future pack updates stay reproducible.

References

Use the documentation for the exact software and release you run. These sources support the settings or diagnostic behavior used in this fix.

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