US election 2016: Who says you can only vote once?

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.

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,