State Farm said Thursday that a glitch in its automated billing process resulted in 80,000 customers, including about 4,300 in Maryland, being charged twice as much as usual this month.

The problem affected customers in a dozen states and the District of Columbia who have monthly premiums automatically debited from their bank account, said spokesman Phil Supple.

The insurer is still looking into the cause of the problem. It became aware of the double-billing Wednesday morning and within 24 hours returned the overpayment to customers' accounts, Supple said.

"If that's not the case, we've asked folks to contact their agent and make it right," he said.

State Farm also promised to reimburse customers if the double billing triggered any charges, such as an overdraft fee.

The other states affected are Delaware, Florida, Georgia, Iowa, Nebraska, Pennsylvania, Ohio, North Carolina, South Carolina, Virginia and West Virginia.

This isn't the only computer glitch for State Farm this year. About 12,500 credit card customers of State Farm Bank in May were shocked to discover erroneous charges of as much as $20,000 on their cards. The problem stemmed from a computer coding error at the vendor that handles the billing, Supple said. State Farm promised to pay customers for any extra charges incurred from the mistake.

eileen.ambrose@baltsun.com