By Jonathan Davies
E.On has been fined more than £7 million after it was found guilty of overcharging customers again.
E.On is one of the UK's largest energy suppliers and was fined £1.7m in November 2012 for the same offence, although it claims the underlying reason was different.
Energy regulator Ofgem described it as "repeated failing" when it issued the fine.
"It's absolutely unacceptable that E.On failed to provide these vital customer protections yet again, and this persistent failure is the reason for the high penalty," said Sarah Harrison, Ofgem's senior partner in charge of enforcement.
Some customers paid too much when switching and others were wrongly charged exit fees. Around 48,000 people are believed to have been overcharged.
In most cases, customers were wrongly charged no more than £10, E.On said.
The fine will go to the Citizens Advice Bureau.
In May 2014, E.On was fined £12m for mis-selling; the largest fine ever handed out by Ofgem.