DENVER, Colorado (AP) -- A computer hardware glitch at First Data Corporation resulted in more than 800,000 Mastercard and Visa transactions being double- or triple-billed last week at Wal-Mart stores around the nation.

First Data spokeswoman Staci Busby said Sunday the excess charges, which occurred on March 31 and were posted April 1, had been reversed. "First Data corrected the problem as soon as we discovered it," Busby said.

Busby said the problem showed up on reports the company regularly generates for quality control purposes. She was unable to say how many customers were affected, but did say Wal-Mart was the only retailer involved.

"Anyone who conducted a transaction with a Visa or Mastercard on March 31 should check their statements," Busby said.

Wal-Mart spokeswoman Danetta Thompson said Sunday the retailer had posted signs about the problem and was working to inform its customers. "As far as the corrections are concerned, it was First Data's system that had the problem," Thompson said. "I feel like they've got their arms around the problem. Those charges have been reversed."

Thompson said Wal-Mart cashiers have been told about the mix up and that signs in the stores point customers to First Data's toll-free number -- 888-893-0626.

Busby said that in addition to the toll-free hot line, First Data was making outbound calls to affected customers. She said that some people may not see the reversed charges on their accounts until Tuesday.

"Consumers need to regularly monitor their payment records, their bank accounts and bank statements to ensure there aren't any discrepancies," Busby said.

She said First Data had instituted additional quality control measures as a result of the overbilling.

"We doubled and tripled our quality assurance measures," Busby said.

First Data Corporation is an electronic payments processor based in Greenwood Village, Colorado. The company owns Western Union Financial Services, Inc. Wal-Mart Stores Inc., is the world's largest retailer, based in Bentonville, Arkansas.

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