Warning: You may be charged twice for parking - and not even know it.

Carpark operators admit double deductions by electronic parking systems (EPS) do happen here, but claim they are rare.

The Urban Redevelopment Authority (URA), which operates 134 off-street carparks, said it had received "fewer than 10 reports" of double deductions at carparks equipped with EPS.

"The errors were due to technical issues and our EPS operators have refunded the affected motorists," a URA spokesman said.

The Housing Board, which operates some 1,800 carparks here, said it had encountered only two such cases in the past five years.

An HDB spokesman said both cases were caused by possible malfunction of the cars' in-vehicle units (IUs).

Wilson Parking, another major operator here, did not respond to press queries.

The low incidence may, however, be because motorists are unaware when a double deduction occurs.

Motorist Winston Foo, 54, said: "The odd thing is the double deductions are not reflected in the transaction slip printed. The only way is to tally the opening balance after top-up and the closing balance."

Mr Foo, a retiree, said he did just that and uncovered at least five cases of double deductions since last December.

"I'd like to know where the missing money goes to and how can it be accounted for by carpark operators or the CashCard company," he said.

The HDB said there were "a few double deductions recorded in Mr Foo's in-vehicle unit in recent months". "To pre-empt similar incidents in future, our service provider has offered to assist Mr Foo in getting his IU checked at Vicom."

The HDB said it will also refund Mr Foo.

Motorists occasionally complain about double deductions on car sites such as MyCarForum.

Retiree Ang Kim Hock, 60, said he discovered a double deduction in 2014 and another last year.

"The carpark operator is aware of the problem and informed me that it happens infrequently," Mr Ang said. "I told them this is not the point... This is a serious breach, and it should be rectified."

He said he also contacted CashCard provider Nets.

"Nets told me that all these unrecorded deductions go into a float account and it will stay there until someone makes a claim," he said. "But how to make a claim if people are not even aware of the double deduction?"

A Nets spokesman told The Straits Times: "Cases of a second deduction are uncommon but may arise if the operator's system did not record the first deduction successfully and may proceed to initiate another deduction.

"As in any payment transaction, customers are advised to check that the correct amount is deducted."

He added that "to deliver greater convenience to consumers", Nets' 2,000 self-service kiosks will soon be equipped to make refunds.

"A consumer who encounters a double deduction can either contact Nets or the carpark operator," the spokesman said. "Nets will investigate the case and if the claim is valid, we will set aside the refund amount and link this amount to the card CAN number."

Nets will then contact the consumer that the refund is ready at the self-service kiosk. The consumer can then place the card into the machine at the station, press the "credit/rebate" button, and the correct amount will be credited back to the card, Nets said.

Nets can be contacted on 6274-1212 or info@nets.com.sg.