Hi David,
Go to the T.code BS22 and select I0184 - ESTO and double click on the line.
Then select PMS1 - Good Receipt and change the radio button to Allowed or Warning and save it.
Warning: Once you activate this setting, you will no more receive any error or need to change the status of the SN, even if the SN already exist in the warehouse.I mean same SN will be allowed for multiple GR transactions, even though the SN already exist in the warehouse.
Best option is to change to warning option, provided this does not Impact your business scenario.