Q:

Mage registry key "_singleton/magenotification/observer" already exists

I am facing a serious problem when I enter the URL in the browser. Firstly the site, not shown then I enable the error log from local.xml in errors file then I found this problem. Help me someone to get rid of this problem.
thank you.

A:

It is possible that u have this module installed over composer + copied them manually on your ftp server? Because that can cause this. Make sure u only have 1 instance of the module installed. In magento 2 this is the app/code folder and the vendor/ folder. Only 1 of the 2 can be used at the same time for the same module.
Try deleting the app/code (not sure what this is in magento 1x versions) module directory.

