Unity alt-tab application switcher in Ubuntu 16.04
Solution 1:
It's due to Unity and Compiz's application switchers both being enabled with the same keybinding (alt+tab).
You can disable the Compiz Application Switcher by:
sudo apt install compizconfig-settings-managerOpen the CompizConfig Settings Manager by launching "ccsm"
Select "Window Management"
Uncheck "Application Switcher"
Now, only Unity's application switcher (Desktop > Ubuntu Unity Plugin) should appear when you press alt+tab.