RBS (LSE: RBS.L - news) has had to issue another apology for last month's costly technical glitch after confirming duplicate loan payments were mistakenly taken from customers at the height of the disruption.
A "relatively small" number of accounts across the country were affected, the company said, in response to complaints.
It is understood RBS and NatWest account holders are involved and RBS said it had increased call centre staff by 50% to handle a potential rise in call volumes.
The bank is urging customers to get in touch if they are experiencing problems and it has promised that no one will be left permanently out of pocket.
A spokeswoman said: "We apologise to any customers experiencing problems today.
"We said last week that we expected to see a few bumps in the road for customers as we get things fully back on track.
"Any customers experiencing problems should contact our call centre or visit their local branch and we will put things right."
On Monday - almost two weeks since the RBS Group first encountered difficulties updating customer balances after the IT failure - it admitted it had taken longer than first anticipated to clear the backlog at Ulster Bank and efforts would continue during the week.
The glitch originated in Edinburgh and was related to old technology still used within the wider banking system.
The resulting scandal meant that millions of RBS, NatWest and Ulster Bank customers had trouble accessing their cash, paying bills and meeting other financial commitments.
Some were even forced to miss holidays while house purchases were also disrupted.
It resulted in Stephen Hester, the bank's chief executive, deciding not to take his bonuses for 2012.
He said other members of the management team would also be accountable on bonuses.
It is also understood that RBS is among several lenders being investigated by the Financial Services Authority over the rate-rigging scandal, which has now claimed the job of Barclays (LSE: BARC.L - news) chief executive Bob Diamond.
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IT Failure Hits RBSRBS has had to issue another apology for last month's costly technical glitch after confirming duplicate loan payments were mistakenly taken from customers at the height of the disruption.