And to be fair, he was telling them in a state where anyone who tried to do so would probably be arrested, or at least stared down. He told his crowd of supporters to do that because - he said - the votes would not be properly counted. His line throughout this campaign has been that the system is skewed against him and that he would be cheated out of a win.
The line would sound shocking if it weren't vintage Trump. The candidate for president has said he will decide on the day whether to accept the results of the actual election. But curiously, the call to "re-vote" is not as crazy as it sounds.
There are seven US states in which the practice is perfectly legal. In Wisconsin, Michigan, Pennsylvania, Minnesota, Mississippi, New York and Connecticut. If you have a change of heart, you can go back and do it again.
There are some restrictions. In Minnesota, it must be done by the week before. In Wisconsin you can only (only) go three times. In Pennsylvania you can change right up to election day itself - but you have to do so in person.
Few people make use of this curious loophole, but Donald Trump wants to exploit it to the full - telling Democrats who already voted for Clinton before the FBI reinvestigation to "go back and vote again".  Presumably he has thought through the ramifications.
It works both ways after all.
Emily Maitlis is presenting BBC Newsnight's coverage of the US presidential election. You can follow her on Twitter and watch more of her reports here,

The former maths teacher captured the explosion of the post-punk rock scene in Scotland as well as the new wave of acts which rose to success in the early 1980s.
Papadopoulos was a staff photographer for the music weekly Sounds, where he became close friends with Scots bands such as Orange Juice, Aztec Camera and The Bluebells.
Ken McCluskey, who was the lead singer of the Bluebells, has co-curated the exhibition What Presence! The Rock Photography of Harry Papadopoulos which is at Paisley Museum until 1 March, admission free.
It includes many of the big names from the Scottish music scene of the time and some who never quite made it as rock stars but had success in other fields.
One photo shows Peter Capaldi, now famous for his role in Doctor Who, as the lead singer of art school punk band the Dream Boys, with Scottish comedian turned US talk show host Craig Ferguson on drums.
McCluskey, whose band had a number of hits including Young at Heart, told BBC Scotland: "My first memory of Harry is seeing him outside the Apollo in Glasgow and he was selling 10in by 8in photographs.
"He would go to Edinburgh and take live shots of a show by David Bowie or someone, develop them overnight and then sell them outside the Glasgow gig. He was quite entrepreneurial."
McCluskey adds: "A year or so later he was taking my photograph when I was in the Bluebells."
A distinctly early 1980s shot of McCluskey's band in a car showroom on Glasgow's Woodland's Road is one of the photos in the exhibition.
McCluskey says: "Harry got a job on Sounds magazine as a staff photographer and he got a flat in London that was a home-from-home for all the Glasgow and Edinburgh bands like the Fire Engines, Orange Juice, the Bluebells, Aztec Camera and Joseph K.
"There would be sleeping bags all over his floor."
According to McCluskey it was Harry's fearless attitude that made his photographs special.
"He was gung-ho and would be right up on stage," says McCluskey.
"He was a guerrilla photographer. In the live shots you get lots of the sweat and frenzy of the time.
"There was no airbrushing. That's why they have lasted the test of time."
Harry suffered a brain aneurism in 2002 and returned to Glasgow in 2006.
He says he is proud of the shots he took.
"I never used lighting or anything like that, I just chanced my arm," he says.
McCluskey says the photos have a feeling of social history to them.
He says that the 1980/81 period was one where Scottish bands such as those on the small independent Postcard record label were taking the UK pop scene by storm.
"It was a time when there was a light shining on central Scotland," he says.
"It has been said that central Scotland invented Indie music."
The first time I heard Donald Trump in Colorado call on voters who had already voted to go back and vote again, I nearly fell off my chair.
[NEXT_CONCEPT]
An exhibition of images taken by Glasgow photographer Harry Papadopoulos is on display in Paisley.