What happens when you hit the Alt key a second time, after you’ve already (1) used the Alt key once to activate the Ribbon Guides and (2) you’ve followed the letters to dig deep into the PowerPoint Ribbon to find a command or feature?
Hitting the Alt key a second time immediately closes the Ribbon Guides (regardless of how far you have navigated into the Ribbon using your keyboard).
Hitting the Alt key a second time moves you backwards in the Ribbon (it undoes your last Ribbon Guide selection).
Hitting the Alt key a second time jumps you to the Search input box at the top of the PowerPoint Ribbon (which assumes you need help finding a command).
Hitting the Alt key a second time does nothing after you’ve already activated the Ribbon Guides once using the Alt key.