Bookmakers Sky Bet, which has offices in Sheffield, has been fined £1 million for failing to protect vulnerable consumers.
The Gambling Commission said the failures related to Sky Bet customers who had "self-excluded" from gambling, a tool used by those who feel they are having trouble controlling their gambling and request that the operator refuse their bets.
However, 736 customers who had self-excluded with Sky Bet were able to open and use duplicate accounts to gamble, the watchdog found.
About 50,000 self-excluded customers received marketing material by email, mobile text or a push notification within a mobile app, and 36,748 did not have their funds returned to them when they closed their account.
Gambling Commission programme director Richard Watson said: "This was a serious failure affecting thousands of potentially vulnerable customers and the £1 million penalty package should serve as a warning to all gambling businesses.
"Protecting consumers from gambling-related harm is a priority for us and where we see operators failing in their responsibility to keep their customers safe we will take tough action.
"Sky Bet reported the issues to us quickly, co-operated with us and has taken this investigation seriously."
Sky Betting and Gaming chief executive Richard Flint said: "We have always taken responsible gambling and player protection very seriously, but this incident showed that we needed to do more.
"When we spotted the issue, we pro-actively notified the Gambling Commission and have worked to improve our processes to avoid this happening again.
"We could and should have made it harder for self-excluded customers to open duplicate accounts with us and for that we are sorry. We fully agree with the Gambling Commission's findings and will donate the agreed sum to charities for socially responsible purposes."
He added: "We want to reassure people that we have not made any profit out of this episode. In relation to account balances, wherever possible and practical we have returned the money to the people involved."