You log into Jenkins. You go to Manage Jenkins -> Manage Plugins -> Available. You see the same plugin listed twice (e.g, under different categories such as "Build Reports" or "External Site/Tool Integrations"). What should you do?
This is a workaround: look at the version of the plugins. Are there duplicates the same version? If the versions are the same, ignore one duplicate as we do not think it will cause a problem. For convenience Jenkins displays plugins in different categories if the plugin is a member of two categories.