It is common for voting rights groups and political campaigners to provide transport to take voters to the polls. Their buses tend to target areas where residents are more likely to support a particular candidate or political party. Repeat trips may be made meaning that voters are taken to multiple polling stations in order to vote multiple times.
Some people are also able to vote more than once via both absentee ballots and in person.
There is not a significant number of prosecutions or convictions for voter fraud based on people voting multiple times at different polling stations.
Numerous USA-wide and state-wide studies over the years have not revealed evidence of major, widespread electoral fraud in the form of postal ballots.