Online Deposit Error

Hubby and I open a stock market account in Citiseconline broker.We chose this broker because every transactions is online.To open an account, deposit,buy and sell stocks, withdraw money everything you want can be transacted anytime.The website is friendly user and their customer service is always open and free to answers all queries and problem.
Last week, I was surprised that I receive a deposit receipt in my e-mail.I checked our account and noticed that there was an additional money credited.I call hubby if he deposit a money but he say's "NO". I tried to contact the customer service of Citisec to inform about the money deposited in our account.The girl insist that money was for us because the name and account number belongs to us.I did not argue.Hubby and I decide to keep the money in our account since we do not really know who is the owner of the money.
After, a week the Citisec office called me in my mobile phone asking about the money deposited in our account.I told them to debit the money in our account because it was not really our money! However, I want to assure the security of our account that this things will not happened again.The accounting officer explained to me the deposit error incident.They cannot track the depositor name that made a wrong deposit because she send a deposit slip using my account number.In the transaction online my account name appeared.The customer who made a wrong  deposit called Citisec office asking about the money deposited because it was not yet credited in the account.The accounting department send me a memo to document and as a proof copy that they debit the money in our account for clear transaction.
The lesson is we have to check carefully the account number before we send a deposit slip to the bank or stock office.