how to recover after the "Item already owned" happened

Some customers of my product have the problem by using the google iap v3 to pay: 1. Signature verification failed for product(response:-1003:Purchase signature verification failed) The item is actually purchased (it appears on Google Checkout and on a 2nd buying attempt it says “Item already owned”)

Any one knows when it happened, how should I tell the customers to resolve the problem that they could be back to the normal.

Thanks for any information.