Accidental dividend overpayment received (opens in new tab)

I have an investment (a VCT in the UK) which pays dividends every six months. The last dividend was accidentally paid to me twice by the administrative services company. They have asked, politely, for me to return the overpayment. The wording of the letter makes it sound like they're asking me for a favour, they don't state that it is required for me to repay it, and apparently they cannot reclaim it automatically. I don't object to repaying it, I haven't spent it and it's not an amount of mo...