Questions have been raised from benefits claimants after the government revealed that 180,000 people are entitled to back-payments following years of errors.
Calculation mistakes were made during the process of moving people from incapacity benefit and severe disability allowance on to employment and support allowance (ESA).
The average rebate is about £5,000.
But some claimants are unsure if they are due a payout.
Who has been affected?
Most underpayments occurred between 2011 and 2014, the Department for Work and Pensions (DWP) said.
It affects those who were not considered for income-related ESA when they were switched to contributory ESA.
These people may have missed out on other payments, such as the enhanced disability premium.
How do claimants know if they are entitled to a rebate?
Anyone affected will be contacted by the DWP and all backdated payments should be made by the end of 2019.
A spokesman for the department said the process of contacting people entitled to repayments may take some time.
There are 400 DWP officials working through cases, with more expected to be employed in the next month.
What if a claimant has died?
Jeanette, from Hull, told the BBC her husband received ESA for two years while he was undergoing treatment for lung cancer.
She questioned whether the backdated payment would still be made as he died in 2012.
“My frustration is that the extra money would have been really helpful at the time,” she said.
“I had to give up work to care for my husband and all our savings went out the window.”
While the DWP is unable to comment on specific cases, it did say in instances where the benefit recipient had died, the calculations would still be completed and any payment would be made to the next of kin.
Will a rebate affect benefit entitlements?
Keith, from Somerset, received a backdated payment of £23,000. He was concerned the lump sum would mean a drop in his benefits leaving him worse-off in the long term.
“I received a letter a couple of weeks ago which said I was owed money,” he said.
“My worry is that I will move on to universal credit some time next year and they might take money away from me.
“It feels like they are giving with one hand and taking away with the other.”
The DWP said any payments under £5,000 would not be considered when other benefit entitlement is calculated – but any payments above that figure would only be disregarded for one year.