The accident of people boarding the wrong train and claiming other people's places is relatively frequent. Another frequent issue are people having tickets for the wrong date. Even more frequent, people seated at the right place in the wrong car. Usually conductors in the end manage to remove offenders from their seats - and remember that boarding a freccia train on a ticket issued for another train does not mean that you are just lacking the seat reservation, it means you are completely without ticket, so you have to pay again the full ticket plus a fine.
Much more difficult is managing double reservations - it may happen and when it happens with groups it may be a major disaster. The poor conductor is not personally at fault but it will become the scapegoat of the railway administration.
I saw once, many years ago, a case of travelers getting so angry with the conductor that he summoned railway police and had them arrested at the first stop.