When returning cryptocurrency that was sent from another Coinbase account, do not return the funds to the original sending address or they will be delivered to Coinbase, not the intended recipient.
Return options
Send the funds to the original sender's registered Coinbase email address or phone number.
Ask the sender to generate a new address or provide an existing address if they have one.
For funds that were not sent from a Coinbase account, ask the sender to provide a valid address.