It is estimated that 5,000 people in Wales who are paying a spare room subsidy could - and should - have been exempt.
Under the rule, anyone who has been in their home since before 1996 and has continually claimed housing benefit does not have to pay the subsidy.
A spokesperson for the Department of Work and Pensions say they are looking at the issue carefully and will take any necessary action.
More top news
Police are warning van drivers to be vigilant following a spate of catalytic converter thefts in the city.
Patients needing orthopaedic surgery in Wales will be asked to lose weight and give up smoking as NHS struggles to cope with waiting lists.
The Prime Minister has defended an RAF air strike that killed two British jihadists in Syria, including Reyaad Khan from Cardiff.