The home heating company Maxol Direct has apologised to 2,200 customers who were charged double by mistake.
The firm has blamed a computer error for direct debit payments being taken twice. It said all customers will be refunded within the next few days.
The heating oil company was recently taken over by DCC Energy.
In a statement, Maxol said it was no longer responsible for customer relationships, billing and payments.
"As part of the deal, DCC Energy Ltd can continue to use the Maxol name for a period of 12 months," it said.
"Maxol is disappointed to learn of this error and trusts that DCC Energy will rectify matters for their customers as soon as possible."
DCC Energy is Northern Ireland's largest supplier of home heating oil with an annual turnover of £230m.
It is ultimately owned by Dublin based DCC plc.
John French, the Consumer Council's head of energy, said the local oil industry needs a system which guarantees standards for customers.
"We are concerned about people incurring bank charges one week before pay day and especially coming into the Christmas period and how they are actually going to cope with that," he said.
"The Consumer Council will once again call on the Northern Ireland Executive to examine whether oil regulation would provide consumers with greater levels of support and confiidence for the industry."