Minecraft-troubleshooting:Mod ID conflict

From Serenity Servers Support

Console Error: Mod Id Conflict. Log: CONFLICT @ [ID] item slot already occupied [BLOCKINFO]

What cause this issue:
Two mods are setup to use the same ID for a item, block, mob or another entity.

If you are using a modpack, remove the config folder and reupload the default one from the modpack.
If you are using your own mods, you need to edit the mods configuration to change the item IDS that are conflicting. Download the folders containing the configuration files, and replace any duplicate in the ID's then reupload and restart.