“…Ralph Thurman, a 72-year-old Republican who Frank turned in after seeing him vote twice on Election Day, once for himself and once for his son, who was a registered Democrat.”
..
“…Thurman pleaded guilty to repeat voting. He was sentenced to three years probation and barred from voting for four years”

The vast majority of people are able to make the personal risk/reward calculation that illegal voting entails, and avoid it like the plague.