Duplicate mods found — the same mod is installed twice

What the log looks like

duplicate mods? found
found a duplicate mod
duplicate mod[^\n]{0,30}id
mod id [^\n]{0,30}is already (?:loaded|present)

How to fix it

1. Two jars of the same mod, usually one with (1) in the name
2. Keep the newer one and delete the other

Check your own log