This has come up a lot lately, as a result, a former municipal clerk, who asks that they not be identified, sent this. I felt it was very helpful. Bottom line; this kind of fraud wouldn't be impossible, but probably pretty tough. Certainly unlikely in droves:

As a former municipal clerk, I have some insight on the voting process. IF a clerk is utilizing the State Voter Registration System like they are supposed to, there is no way to "double vote". The system prints out the absentee ballot labels and the clerk is prompted to enter in the return date of the absentee ballot when it is returned to their office. When the poll list is printed out prior to the election, the list clearly states the voter already voted by absentee ballot. BUT not all clerks run the absentee ballot process through the State Voter Registration System like they should.

The second "fail safe" in place is that prior to the open of election day voting, the clerk gives the absentee ballots to the Chief Election Inspector to check in prior to the open of the polls. The names of those voting by absentee ballot are hand entered into the voter list. Once the polls open and a lull in voting occurs, the actual ballots are opened and processed through the machine. BUT do all absentee ballots get entered prior to the open of voting, no. Is it possible for duplicate voting? Yes. BUT the person voting twice is knowingly committing a felony. And the double vote would eventually be caught by the end of the process.

Could an unethical clerk or poll worker discard ballots they KNOW were made by a conservative or liberal  voter based on their leanings? Yes. But they are knowingly committing a felony. Voting "day of" is technically more secure than absentee voting. Less variables. Less chance for a return ballot to be "lost in the mail".  It is also your only way to correct an error on the ballot and NOT have the entire ballot rejected.

The "problem area" is actually absentee voting by those in nursing homes. People's kids and grand kids "help" them vote. Nursing home employees "help" them vote. And even sometimes poll workers "help" them vote. This is all illegal, but it happens more that you think.

So there's my 2 cents. Hope it helps clarify things.