Some states interpret that a submitted absentee ballot has been cast, and you can’t “un-vote”, not least because it’s anonymous and so there’s no way to know.
If you vote in person, drop your ballot in the box and then keel over, they don’t fish your ballot out of the box.
Other states interpret a submitted ballot as having been filled out and collected, but not yet cast. They’re usually kept in the sealed envelope with voter identification on them until they’re opened and anonymously counted when voting begins. They check names against the voter registry as they process them, and there’s a process for double and triple checking things to ensure there’s been no error (where I am they try and call you or otherwise get in touch with you, and if there’s any doubt your ballot becomes provisional so they can dig in if it’s close enough to matter).
If you vote in person, and keel over before putting your ballot in the box, they don’t pick your ballot up and drop it in for you.
It looks like Georgia falls into that first category.
It’s a vanishingly small edge case so it doesn’t get tested that often.