App icon duplicated with a 'grayish' overlay on iPhone 6 with iOS 8
This happens sometimes when an app installation or update fails and SpringBoard still shows the icon from cache.

To fix it, try any of the following:


Sometimes, holding down on any icon to start edit mode, then tapping the × to delete the placeholder will work fine.
Reboot—this clears SpringBoard cache.
If your device is jailbroken, run uicache as mobile, then respring.