alt_name=Carnarvon does not seem appropriate https://www.openstreetmap.org/node/29422113
According to the linked Wikipedia article  "Carnarvon and Caernarvon are Anglicised spellings that were superseded in 1926 and 1974 respectively." 
May be better to use old_name:en=Carnarvon;Caernarvon similar to what has been correctly done the old old anglicised Conway spelling of Conwy https://www.openstreetmap.org/node/29269859