Owing to a genuine error, one of my parents turned up to vote after sending in a ballot in the post.
He was politely told he had voted already. They knew, because as the postal votes were returned the name of the person whose vote they were was crossed off the list of people eligible to vote in that polling station.
I can see it as a way for someone of evil intent to get rid of ethnic minority (or in Flint, Michigan, majority) voters, though: "no, sorry, you already voted by post" might well confuse some, I should think, and even if it didn't they wouldn't be able to do anything about it if there was no ballot paper available for them.