Customer paying VAT twice?

Hi,

Received this message today. What do I do here? Are they correct? I'm unsure how to answer as never had this questioned and to be honest, I don't understand how it all works now at all.

"Hi you charged VAT on the item, yet I had to pay VAT at customs in Ireland, therefore the VAT you charged on the invoice should be refunded. VAT customs charge receipt available"

Small value item <£20. CN22 with IOSS number also on the package.