SINGAPORE - More motorists have said they were charged twice for parking, following a report by The Straits Times Monday on the matter. At least half a dozen motorists said on the paper's Facebook page that it happened to them, too.

Facebook user Tay Peng Tong said he was double charged "five times in 2014". He claimed that he had his car's in-vehicle unit inspected but the problem persisted.

"Now I...slot my CashCard into the machine instead of leaving it to automation," he wrote.

Carpark operators said they would not know a double deduction had occurred unless a motorist complains.

But 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."

So, many more consumers may have encountered this phenomenon unknowingly.

Electronic parking system developer Nets and Mitsubishi Heavy Industries did not respond to queries on whether their systems know when a double deduction occurs, and if they did, how many cases of double deductions they have come across.

Nets would only say "cases of a second deduction are uncommon".

The e-payments provider did not say what happens to funds arising from double deductions that are unclaimed.

Mitsubishi, which recently clinched a multi-million-dollar contract to supply a "gantryless" electronic road pricing system that has the ability to charge for streetside parking, was not able to reply by press time.

Said Facebook user Vincent Ng said it was highly improbable that the companies' systems cannot detect double deduction discrepancies, adding: "Are you telling me that there are no check and balances in place in this companies?"

Mr Foo meanwhile, said: "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."

Another motorist, 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?"