Possible Cause: Duplicate module folder

Possible Cause:
There is one or multiple copies of the same module folder.

Fix:
For example in my case the error returns Fatal error: Cannot redeclare system_requirements() (previously declared in /modules/system/system.install:12) in /modules/modules/system/system.install on line 299

1. Search for duplicate(s) module folder thats triggering the error. In my case I search for a duplicate system folder.
2. I found two folders with the same name system. The first one is located under modules/system. And the second one is located under modules/modules/system
3. To fix this delete the duplicate folder(s). In my case I delete the system folder located under modules/modules/system

If above fix isn't working try the same above steps but search for duplicate(s) inside the other modules folder located under sites/all/modules