Southend-on-Sea City Council has contacted residents affected by an error in care cost calculations.
The error, which dates back to 2018, was caused when a new software system used to calculate care charges was configured incorrectly.
This led to some people being overcharged for care in their own homes or while living with family members, supported living, shared lives, extra care and day care.
A total of 586 people have been contacted, including 335 people who currently receive this care.
Cllr Daniel Cowan, Leader of Southend-on-Sea City Council, said: "We recently identified a technical error, originally made in 2018, that has caused some residents to be overcharged for their care.
"On behalf of Southend-on-Sea City Council, I sincerely apologise for this error. I am pleased to report that council officers have moved quickly to assess its impact, and we are putting things right.
"A dedicated team has reviewed every affected individual's circumstances. We have contacted each person to make sure they get the refunds they may be due, and we are confident that we now have the systems and processes in place to stop this happening again."