Does anybody have an explanation for this?
There could have been a technical problem with the account handling at the service you were using, where customer service saw your request and decided to add funds to your account, and then the technical problem was resolved and you got credited a second time.
Or your ticket was looked at by two different people doing customer service, and both gave you credit at the same time, before the support ticket was closed.
Or the outfit is so sloppy they just honor any complaints and pay out to resolve it. This is particularly true for sites that hold a wallet (e.g., exchanges, escrows, wallets) that are fraudulent. If they don't really hold all the customer funds that they should be holding, then who cares if the proceeds of the fraud is only for 80% of the balance (because 20% was credited to fix problems like this).
There are a lot of reasons this could happen. And each time it does should be a warning sign.