Thousands of California voters were sent duplicate mail-in ballots ahead of the midterms as questions swirl over the security of the state’s election process.

The error occurred when a printing company that has handled Tulare County’s ballots for years mistakenly doubled up, confusing the nearly 8,000 voters who received the forms.

About 3% of the county’s 227,000 registered voters were affected. In the Republican-leaning city of Tulare, which has just over 34,500 registered voters, roughly 23% received duplicate ballots.

The confusion spilled into the registrar’s office in Visalia last week, when residents called and showed up in person, worried they could be accused of attempting to vote twice.

Mario Garcia discovered the mix-up after checking his mail on two separate occasions.

“I waited a couple of days to pick up my mail, and I picked it up on Friday and I saw one ballot, and yesterday I went again to get the mail and I saw the other one,” Garcia told ABC 30.

His wife and son also received duplicate ballots, he said.

County Registrar of Voters Chief Clerk Michelle Baldwin said the printer, Porterville-based ProVote Solutions, had accepted full responsibility for the mistake, while maintaining that safeguards were in place to prevent the extra ballots from being counted.

“Accidents do happen,” David Becker, executive director of the Center for Election Innovation & Research and a former Justice Department voting enforcement attorney, told the Los Angeles Times.

“Our elections are incredibly decentralized, run by hundreds of thousands of human beings. As a result, we know there is going to be human error, but our elections are not designed to be perfect. That’s impossible.”

And Tulare County was not alone. Imperial County officials announced Monday that 160 voters with confidential status had received duplicate ballots in a separate mailing error linked to changes in ballot processing procedures under California law.

The incidents came as federal officials announced charges Monday against 28 people in California involving election-related crimes, including alleged fraudulent voter registration and false statements on naturalization applications.