Found duplicate mods Forge server

What this actually means

Two jars in mods/ declare the same mod id — usually an old copy left behind next to the new one after an update, or the same mod shipped inside another jar.

How to fix it

Delete the older jar. Sort mods/ by date and look for two files whose names differ only by version. If you deploy by copying a new jar in, ALWAYS remove the old one first -- copying does not replace a file whose name changed.

How to be sure it is this

List the folder sorted by name and look for two files sharing a mod id with different versions.

What it gets mistaken for

Forge loads anything ending in .jar, so mod (1).jar loads and mod.jar.bak does not. The copy with a number in brackets is the usual culprit.

What to do next

Keep the newer jar unless a dependency pins the older one. Check .disabled and backup folders too — a lot of hosting panels move rather than delete.

What it looks like in your log

Search your crash report or latest.log for any of these:

Found duplicate mods
DuplicateModsFoundException
Duplicate mod ids

The free edition of Forge Server Doctor detects this one. It is a single Python file with no dependencies — point it at the report and it names the cause and the mod jar:

python3 forge_doctor.py crash-reports/crash-....txt