Very easily done. I've done this and not knowing the official answer, and because the buyers were each happy to co-operate to remedy, I just bought new labels and sent the QR codes to each.  I figured this removed GDPR problems in that neither buyer then had access to the other's details.  All went swimmingly and both buyers left nice feedback and have bought from me again since.

Having two buyers who actually want their item and are happy to help sort the situation helps; this could have all gone horribly wrong if one items had been lost in the post, but as a solution it was quicker than getting the items returned and then resending them.