Try deleting modules/_protected folder in your web project and then reinstalling the nuget packages to bring them back in a clean fashion.
Anything I suggestion are going to be shots in the dark so ultimate it might be prudent to reach out to support.
However you mentioned installing an addon...sometimes when an addon is uninstalled, some cleanup tasks get missed. Maybe have a look at this addon's source code and see if you can find a class that inherits from PackageInitializer. That'll show you what actions it performed when you installed it.
Found in EPiServer.CMS 11.20.14 (EPiServer.CMS.UI 11.36.7)
I have two issues with duplicate entries of 'Pages' tab and 'New Block' entries in Optimizely CMS interface. I first suspected that this was related to a plugin that was installed but once I removed it the behaviour still existed. Has anyone experienced this kind of behaviour or has any idea how to remove the duplicate entries?
The Pages tabs hold exactly the same content but the New Block entries correlate to the following popups:
Appreciate any ideas or leads to solve this.