Duplicate payments and tax

Hi,
I am a freelance editor and have been paid twice for the same invoice, once at the end April and once at the beginning of June, meaning that I have been overpaid by over £2K.

I notified the company of the error when I spotted it yesterday and (understandably!) they have asked me to transfer the money back. This is a company that I work for a lot and I currently have invoices with them that are due for payment next month, to a value of more than the overpayment.

As a very new freelancer, I'm a bit concerned about the the possible implications that returning the money will have on my tax. Is there any possibility that this will be an issue? As long as it is clear from my bank statement that the money has been returned, I won't be expected to pay tax on it when I come to do my tax return?

If there is any possibility of it being an issue, would it be reasonable to ask the company to offset this payment against future payments to avoid this possibility?

Many thanks