What kind of illegal voting obstacle was used by the group of citizens who voted more than once? Why would they use it?

The illegal voting obstacle wild be called repeat ballot rigging.

The citizens would use it for political advantage.

Explanation:

Repeat ballot rigging is the phenomenon when one citizen votes more than one time in the ballot and thus makes 2 or more votes count for one citizen skewing the results of the election.

This is commonplace in many democracies and usually is done by instigation of parties who want favor with the vote and have dedicated citizens who would do something import to get them tot he power they are vying for themselves.

These people are usually ardent supporters of the rule of their party.