Identify the duplicate
Read the loader message and inspect the active mods folder for more than one Placebo main jar. File names with different version numbers often reveal a failed replacement during an update. Also check that a sources jar was not accidentally installed for normal play.
Move obsolete files out
Close Minecraft or stop the server, then move old copies to a backup folder outside mods. Renaming a file while leaving it in the loading directory is less reliable than removing it from that directory altogether. Keep the one release required by your tested pack.
Relaunch and inspect the next message
If a duplicate error disappears but a version-range error remains, compare the surviving file against the dependent mod's requirements. These are separate checks. Do not add the old jar back alongside the new one; select a single compatible release instead.