Why do I have two Alt+Tab application switchers?
Solution 1:
Run ccsm from the terminal, or search the dash for Compiz and open CompizConfig Settings Manager. If it's not installed, run:
sudo apt install compizconfig-settings-manager
In CCSM search for Application Switcher and uncheck the box beside it. The change should take effect immediately after you uncheck the box.
No idea what caused the problem in the first place.