Accidental dividend overpayment received

It's a general principle of law - at least in the UK but probably in most jurisdictions - that if you receive something you are not entitled to you can't just keep it.

There are lots of possible exceptions, for example you might claim "change of position" where you genuinely thought you were entitled to it and have spent the money on something you wouldn't have done otherwise. Or when a company sends you goods unsolicited there are stricter rules in the UK to avoid it turning into pressure selling.

It's unlikely any exception would apply here. Since it was their mistake and will cause you some hassle you could in theory demand an admin fee or similar but that's unlikely to be worth the effort or be particularly reasonable in the case of a simple one-off mistake that they have quickly corrected.